My understanding is that no adapters are required. The OHC block will bolt to an M40/41 bellhousing, (giving you an upright engine) and you use a B20 oil pickup tube and pan. The only thing you would have to fabricate are motor mount brackets.

However, I tend to agree with you (and everyone else) that a good strong B20 is all the motor a PV will ever need. Consider that the first PVs had about 40 HP, B16Bs were 85, B18As not much more than that. A stock B20B is 125 HP, already more than TRIPLE the rating of the original B4 of the earliest cars. If that's still not enough for you, there are hot cams and other possible means to boost the power of the B20 even higher. Remember too, that the PV only weighs about 2200 lbs., so big horsepower numbers are not required for impressive performance.

If you are going to go to the time and trouble of major modifications, upgrading to disc brakes (at least in front, if not all around) is a much more worthwhile project than an OHC conversion.