No issues with the engine swap. Both cars should have the same style clutch linkage (later model cars that came standard with a B20 usually had a clutch - yours should have a hyrdraulic). The B18 and B20 are mainly just the same engine with larger pistons - they are the same on the outside - except for a few more bolt oles for mounting things on the B20.

You will most likely not have any problems with the mixing and matching of the manifolds - although there is a possibility that the thickness of the manifolds won't match - and there are four bolts that use thick washers to hold both manifolds at once - you might need some half-washers there. But usually you find issues like that with later model injected manifold s- you should be OK. The SU's (properly set up) are better than the Weber is.

If the shifter swaps with anything else I wouldn't know what it is - it's a Volvo part as far as I know. You must have an aftermarket AC unit inside that sticks out - mine has a factory (actually dealer installed) A/C unit and it in no way comes close to interfering with the shifter.

You could pull the shifter off the trans and possibly bend it back a little (remove all the ruber and plastic and heat it probably). If you wanted a longer shifter you could probably get a l-o-n-g 122 shifter and shorten it down to whatever length you desire. If the bottom section doesn't work (for some reason I have it in my head that the bottom ends of the short and long shifters are the same) you could just swap the top end of the shift lever (the top chromed piece is separated from the bottom - held on by a circlip and some rubber bushings).
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I'm JohnMc, and I approved this message.