You can fit a B trans onto a A flywheel housing with a adaptor plate between the two.
I have just fitted a B trans to my 30 with V8 gears inside , you will also have to make a pedal mount bracket & park brake bracket , also if running a stock wishbone set up there will have to be a mount welded to the bellhousing for the ball pivot .
The only problem that I had was with the starter motor I had to use a early style 1/2" shaft without the bendex spring as it was hitting on eather the plate or the bellhousing ( not sure at this stage ) but will find out later on as I wanted to get it running before going back to work.
Allan.