First I'm not a mechanic just try to give U an opinion base on my limited knowledge. Second, most of Volvo's engine is robust and can last a veeeery long time. It is common and easy to reach 300000 miles.

Now regarding your q If the engine type is b234f in the 740 then i think u have an ok engine. IMHO, b230f or ft is the best. Probably the previous owner didn't give a proper maintenance so it looks like the engine have allready in a not so good condition when your father bought it. As far as I know the engine doesn't have a bad record. The V6-24V(B280 if i'm not mistaken) is the bad one to have. It's possible you may need to overhaul the engine.

If your father's 940 engine is the b280 or b6304, yes you might have problem but with proper maintenance i think it can last a long time. I don't have any idea about the advantage of putting your 87 245 engine (B23 I Iguess) in your 740 except you finally can get rid of the 245 dl and collect some money by parting the remain of 245 and the 740 engine.

Goodluck and keep the I-roll brick rolling. (Just keep the engine oil clean,regular PCV cleaning and use Volvo or Man filter)

david 91'740 Tic (b230FT)