Mmm, Imp... seems a grand machine by the classic car mag i was browsing earlier

alongside Alfasuds and the like. Super lightweight, goes damn well for an old skool 900cc job...
As a 4-speed driver I'd definately reccomend the 5-speed shift, i've more than once wished for it (and come within a gnat's testis of getting one put in, but for a theiving bunch of con artists who still haven't refunded the bloody fee).
But, I say, it would pay to find out exactly which 5-speed it is! The "CL" type is more recommended (the most accurate way of describing it, "regular 4 speed, plus cruising overdrive"). The closer ratio types are more suited if (and only if??) you fancy doing a bit of racing or setting a super time down the twisties as on a long haul you wont be able to go as fast as even the 4-speed in either 4th or 5th, and it'll be a noiser, thirstier experience in both of those compared to the CL type.
*My reasoning -
In the 4-speed, 4th is pretty much cock-on for getting the best speed out of the lower powered cars without destroying the engine or being super inefficient. At full throttle in the 1-litre (low 90s), you'll sit nicely in the lower regions of the peak power zone. Lower gearing would see maybe a half mph extra but a lot more revs (though it
would hold speed better uphill)... and with low-30s mpg! In the 1.3, it'll be in the slightly higher reaches (nearer 100), where it's starting to tail off, so you won't knacker it so much there either. It's nice for cruising round town, too, can just manage 25mph without being too uncomfortable, and do 35, 45-50 very quietly and thriftily. However it's not great for sustained high speed travel, being more of a city-car setup. It sees 70mph at about 4000rpm, and you can cruise most nicely at just over 60... or 75 if you turn the radio way up. Going full pelt for more than 20 straight miles is more of a rare, aw-crap-im-laaaate thing where you turn the radio off and listen to the motor sing, for anything that might be pinging off it under stress.
The wide 5-speed keeps those same 4 gears, and bolts an overdrive set on the end (quite literally - same housing, with a reshaped cover plate and longer spindles), giving ~20% lower revs at the same road speed. You won't go quite as fast flat-out with either engine, but can still sustain decent motorway speeds (80-85, 1-litre full throttle, ~5mph more for 1.3?) on the flat and slight hills, have a lot less noise, and better economy - particularly if you cruise slightly slower at part throttle (say, 70-75 again, but not needing the volume so high).
If you can stand toiling in the middle or left lanes occasionally on steeper inclines, you can probably go many lightly-loaded extended trips without having to change down to keep moving (though speed may drop worryingly!). Even the 1.0 should torque along down to low 60s or high 50s in 5th and not labour badly... unless you're facing Shap or something.
That gear however is probably not at it's best around town, at least not til you hit the 40mph limits. Not enough revs to make power, to sustain the speed you'll be going at, without using a lot of right foot. Therefore you can still truck around comfortably as if you're in a plain 4-speed whilst urban (still getting great mileage even so), but retain a bit of peace at speed - and the option of using 4th for that extra 5 or 10mph at the expense of high revs (how's 5200rpm at 91mph grab you?), noise and petrol.
(( some of the 1-litre 5 speeds seem also to have a slightly different final drive that makes each gear a tiny touch 'lower', having about as much effect as e.g. my change from 145/80 to 155/70 tyres, about 4% - subtle, but a noticable difference. Revs and consumption are slightly higher for a given speed, in exchange for better acceleration, 4th holding speed better, and 5th is a little faster and a little easier on the engine uphill without getting too much noisier... still being 16% over 'original' 4th ))
The closer set 5-speeds however are a bit of an odd bunch, probably more suited to the more powerful engines like the GT. There's one extra close job (1-litre only) where 1st is same, 5th is identical to normal 4th, and the others juggled to fit. Good for the urban delivery warrior, mountainous valley resident, or 1-litre country lane samurai who still wants some motorway prowess. Odd, but not as kooky as the others.
The "GT" style gearboxes are a bit strange, and arguably only a good match for that higher-revving, narrow ranged, more powerful engine, and not so great for the lower powered ones (or the G40) - though you may well see something with similar ratios bolted to a 1.0! It would at least be ok for the 1.3 (5th seeming like a regular 4-speed top does to the 1.0), and motorway driving will be slightly better on the ears and fuel than the 4-speed, but not greatly so (and slower!). It'll seem buzzy for a 5sp!
It's first 4 are quite low-set and close, compensated by a much wider jump between 4th and 5th than might be expected, more than 25%. I don't know if you might already see this is a problem for the 1-litre. Top is almost 1mph per 1000 revs higher, making it hard to hit that engines power peak and get the best speed (or hold what you have), but you're almost out the other side of it at the same speed in 4th and making some very hard revs. That lower edge of it you might have seen in the regular 4-speed - to change up into it in 5th would require bouncing off the rev limiter in 4th. Having to swap between those two gears may become a bit of a tiring experience, particularly as the gap is so wide and jarring. (Even gear-hunting on rolling terrain with the wider box wouldn't be *so* bad, as you probably wouldn't bother to do it until dropping past 75mph, and the difference between the engine speeds would seem much less marked despite the power difference)
Plus its still probably not that good around town
(( actually, the G40 boxes are a development of the GT ones - same core ratios (with a mildly and mysteriously tweaked 2nd) but with a much altered final drive - making each gear about 20% taller... so basically 5th is a little higher than that in the CL, and everything's squashed upwards to match. However seeing as the engine is more than twice as powerful as the plain 1.3, it can cope with a somewhat higher 1st, and the comparitively tall 5th! ))
And so on and foo.........
(I make this argument too often i know

)