You can use the original radiator, but to do so you'll want to find an early B18 thermostat housing - this one will fit the B16 radiator hose easily. Connecting the lower hose is more of an issue - you'll need a long piece of coolant hose and/or a number of 90 degree elbows to connect everything up. It can work but it's not at all graceful.

A radiator shop can move that lower hose fitting from the driver side of the radiator over to the passenger side, and then all will be well. The radiator itself, unless it's compromised, can keep a B20 cool. Unless you're in Texas or something, then I don't know what happens.

The engine mount brackets from the B16 bolt onto the B20, and that's the ONLY factory engine mount bracket that will allow you to simply install the engine. B18 544 brackets don't work, 122 brackets don't work, 1800, 140, 164 brackets don't work.

The bracket for the clutch linkage that's welded to the frame member is in the wrong place for the B18/20 setup. You can cut it off and reweld it; or you can use a heim joint bolted through the flange on the frame member. Heim joints are pretty nice, as they don't scrape and grind like the original stuff.

Once the B18 is in place, you'll notice that the starter is directly above the brake master cylinder, and that you can no longer check the fluid level without first removing the starter. You can either live with it like that, or you can switch to a later master cylinder with remote reservoir; or you can get an adaptor cap for your current master cylinder which will then allow you to add the remote reservoir and associated plumbing (this is the route I'd go).

If your current transmission is a 3 speed, the driveline won't be the right length for the new M40. If it's a 4 speed, the length will be ok and you'll need only worry about whether or not the flange is the right size.

Cheers -

Cameron
Rose City