Jon,

The B21FT has a lower compression ratio than the normally aspirated B21A/F engines do. I can't recall if this is acheieved via the headgasket or if the B21FT actually utilizes different pistons, etc. The B21FT head also has sodium-filled exhaust valves, which are said to dissipate heat better. And, you would have to tap your block to accept the oil return line from the turbo.

Another complication: The 240 turbos had Bosch K-Jet fuel injection, at least in the U.S. I'm sure that there are plenty of carb'd turbos out there, but I dunno if your existing carb. would provide enough fuel or the correct mixture for a turbo application.