The tunnel on a PV is very snug. Of course, that being said someone here not too long ago was showing off pictures of their PV with a Pontiac V6 sitting in the front (and a steering column with lots of little u-joints reaching around the left exhaust manifold!). And people have also put larger V8's in too. I think it helps that the front structure of a PV is simpler than the later cars - practically like a body-on-frame car with two simple frame spars and a bunch of non-structural sheet metal.

PS - You could drop in a later model Volvo B20 from a fuel injected car that would have attachment points for AC compressors (although you wouldn't want to use one of those old style York monsters) and most of the hardware needed to adapt it to a modern engine management system like Megasquirt.
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I'm JohnMc, and I approved this message.