The M41/OD uses the exact same crossmember as the M40. Normally it's an easy bolt-in, requiring only a little wiring and shortening of the drive shaft. With your modified crossmember, I can't say. The OD adds about 40 lbs behind the mount, so you'll need it to be even more solid.

The B20F in that 240 can be easily converted to carbs. Retaining its K-jet injection is a much harder trick, although it's been done.

Most '75 240s used the M40 or M41. If it's an M41, no reason you can't use it -- your existing shift lever and top cover swap right on. It has a slightly lower first gear than earlier M41s, but that shouldn't bother anything in daily driving.

A few used the M45/M46 with a "year-only" bellhousing. This is worth real $$$. have a good look at what's in there...