I tend to think that the time and effort spent modifying the car to fit another engine would be better spent tweaking a B20 to provide a similar power/weight ratio. It seems like cylinder head modifications are the most important factor in upping the power (B18/20's have an unfortunate exhaust port design), which then require enhancements to the intake and exhaust flow. 170 or 180 can be had fairly reasonably easily without forced induction, and there are supercharger kits if you wanted a bit more (bear in mind, of course, that you can't just slap a supercharger on an engine already modified for non-boosted performance - the compresion ratio and cam need to be quite different).

The PV is pretty lightweight - I think around 2250 lbs from contemporary road tests. It doesn't need too much power to move quickly. And the handling already tends toward understeer with the Volvo lump up front, I'd be pretty careful about adding more weight up front (although it is certainly possible that modern aluminum blocked v6's and even some v8's might weigh less than a B20).
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I'm JohnMc, and I approved this message.