The 88 240 will essentially be a plug-in, but you will need to make one significant modification. You need the 740 distributor on the head. Take yours off the original engine, and install it to the 240 engine after removing the cam plug. You'll also need to block off the hole where the 240 distributor lived. Volvo uses something like a freeze plug for this hole. It's likely the original plug is still available through the dealer, though I've never needed or ordered one.
The engine temp sensors and things should all be in the right locations. You'll need to completely remove and swap the accessory drives: alternator and power steering pump are reversed from a 200 to a 700.
Good luck!
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Rob Bareiss, New London CT ::: '87 244DL/M47- 221K, 88 744GLE- 202K, 91 244 181K, 88 244GL 145K