Get a donor car. B21FT requires less wiring work, and is tougher than the B230FT, though a less refined fuel system the k-jet is surprisingly effective at delivering decent power (just crappy mileage). You'll need exhaust, fuel system, wiring harness & ECU/CIS module, overboost switch, etc, hence the need for a donor car to simplify life & keep costs down. A few hundred dollars or less should buy a running but terminally rusted (or smashed) 240 turbo, versus numerous trips to the scrapyard. As for the transmission, you can use your m46 on any red block, bear in mind the turbos use a heavier-duty clutch with a recessed flywheel, but you can use your existing flywheel & clutch assembly. Just don't expect it to last long if you like aggressive launching. Mine held up that way for several years of reasonably conservative driving though.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina