Re: 5sp
the H41 (most likely) in your BJ40 and the H55F have almost the same ratios but thr 5th overdrive gear in the H55F. Also, the 5sp is longer than the 4sp by 60mm or so IIRC, and you would need to relocate the crossmember, and modify your driveshafts accordingly... also I'm not sure you can bolt an early T-case on the back of the 5sp, so you would need to swap both cases in... Also remember that 40 and 60-70 series don't have the same shifter, so you need a BJ42 tranny if you don't want to have the shifter coming right through the heater box...
IMHO it's not really worth the effort. bolt a turbo on the engine, as well as an ontercooler while you're at it, and put some bigger tires for overdrive, and you should have a decent setup...