Hi Artur,
If you've found a decent turbo engine for reasonable money, I'd say go for it! There are plenty of JB 'boxes around should you need replacement... ;D
If/when buying an electrical fuel pump for the Turbo, remember that you need an 'industrial-grade' (read your Gibsons?)
pump capable of building up over 1 bar of pressure. The price for such a pump is in the region of appr. €150-€200 from Pierburg, Airtex or similar good quality manufacturer. You can get electric fuel pumps for appr. a third of that, but they're usually only capable of ca. 0.3 bar and should only be used on atmospheric carburettor engines.
Then there's a fuel pressure regulator in the engine compartment. The actual fuel pressure to the carburettor is reduced to only about 0.25-0.3 bar, but the fuel pump needs to have plenty of 'reserve' when the turbo kicks in requiring extra fuel. The 0.3 bar pumps simply aren't up to the job, causing poor performance and potentially burned valves and/or pistons as a result of heavy stress combined with lean mixture.
Can't remember if the 11 Turbos have the pump fitted separately under the car or if it's been integrated to the fuel level sender unit in the tank? However there's a separate fuel filter under the car, similar to those fitted to fuel-injected cars. That I DO remember...
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The Turbo has ventilated front brake discs (20 mm thick) that also require different calipers. Later Turbos had rear disc brakes, but early cars had ordinary drums, so you can keep your existing rear drums. Also the early Turbos had a 2-bar rear axle, so no need to change that either.
I don't now how strict it is in your country, but at least here in Finland we need to uprate the chassis and brakes to equal the original Turbo if we want to keep things legal. Well, I've seen people remove and bypass the turbo temporarily to get through the MoT without any modification inspection or paperwork needed, but it doesn't help much if the police stops you on the road and you have a turbo but no mentioning of it in the car's papers...
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I personally haven't seen a C1J-760 or -770 been mated to a JB1. Bolt patterns should be similar so it's possible to physically mate a turbo engine to a JB1. IF there isn't place for the 200 mm clutch inside the JB1 bellhousing, then you can always use a standard 160 mm (or is it actually 180?) clutch, although that will probably have a relatively short life. I haven't seen JB1 and JB3 bellhousings side by side, so I'm only speculating here. (Aren't I always?)
;-)
A friend of mine used to have a Renault 8 with a C1J-760, upped boost and about 150-160 bhp. He used a standard R5 Alpine clutch (can't remember if it's 180 or 200 mm) and it slipped when flooring it in 3rd or 4th. Before that he'd used a 160 mm sintered racing clutch which didn't slip. However its 'on-off' function was a bit tricky on ferries and such when he had manouvre in tight places, so he changed back to a more civilized version. He didn't operate on the standard 105 bhp max boost, so I can't know how the clutch would've coped with that 'little' power...
I should also mention that the original R8 4-speed 'box(es) had a hard life connected to the engine; When he sold the car a couple of years ago he was on his third gearbox! In the Eight the weak point were the small cogwheels at the differential. For the (completely different) JB1 it's probably the differential plus the 5th gear bearing, I'd suspect.
If you use the JB1 with the turbo, you should definitely avoid 'Banzai!' starts (revs up, clutch up) and avoid using the torque of the engine in 5th gear. With those precautions, you may achieve tens of thousands of kilometres before you'll a new gearbox. Then again, you might forget all those precautions and blow it when that six-pot Bavarian Motor Wunder stops at the traffic lights beside you and you need to show who's who...!