If I were to put a new engine in my '76 242. How do you know what engine will mate with it well enough to work?

 

Well... some details might be useful here. Are you looking to put a V8 into a volvo, or another red block? Or are you looking to do a NAS to turbo transition?

 

a 1976 is a very early 240, it probably has a B21F, with K-Jetronic injection?

if you wanted to use the newer LH injection that comes with a newer B230F type motor, you'd need to replace the fuel pump, the engine compartment wiring harness, the air paths, both intake and exhaust manifolds, ignition systems, etc. you'd need to mate that engine wiring harness with the dash harnesses as there are various detail differences, fuse panels wired differently, etc. The details would vary with whether that was a LH2-2.2 motor from a 83-88, or LH2.4 from 89-95, and also if the motor was from a 240 or 7/9 (harness were fairly different even if the basic motor was the same).

You probably could get that k-jetronic working with a newer B23 or B230 block, you'd have a lot of tinkering to do, and I don't think the manifolds will just bolt up either, but I might be wrong. I also don't know if the K-Jet and its injectors from a B21 (2.1L) would deliver enough fuel for a B230 (2.3L).

If you're in a smog controlled state like California, you would have to get the conversion 'referreed' by a state inspector, and ALL the smog equipment from the newer car would have to be installed on the older one, and forever more the car would be smog tested as though it was the donor year.

 

Going red block to red block......?

Cyl head: any up until 84...with the 130 cu in on the fender label...no mods just move the temp sensors and plug one or two...intake side....thermal time switch still in the back.
Block...: any red block but as the newer models had certain areas you would have to unplug or plug....I don't have a breather box but a flame trap atop of the cyl head....on my 87 wagon I had a breather box/vent whatever you want to call it....Volvo dealer called it breather box...behind the intake manifold that you would need to take out and install the brackets for the air meter unit...stuff like that..

But you could slap on a 740 turbo model with major tinkering and like Pierce said...smog issues are a pain...you would probably need trans as well or, or just the Bell housing with certain transmissions...or keep the 740 trans but would need trasn cross member support and shaft..etc.