For what it's worth, the Regina engines are among the highest compression of the redblock volvos. 10.7:1 I think. It's in the manual in the engine specs.

Anyway, I replied to your other post and I think you just have the wrong type of O2 sensor in there (for one thing). Regina cars also require a lower pressure Fuel Pressure Regulator (SMP PR80 at 38psi not 45psi) but people do interchange them. (https://www.rockauto.com/en/moreinfo.php?pk=47133&cc=1288425&pt=6124&jsn=954)

If you didn't replace your spark plugs, you really still should. The problem with prying on the center electrode is that it can invisibly crack the ceramic and then it can shatter when heated up.

I've been looking for a reason to learn more about the Regina system since I daily drive one in my 90 740GL. I believe it has some slightly different pattern to the flex-plate's empty/full holes used to determine TDC.

You could also drill some shallow dimples into the accessory pulleys on the crankshaft, the oil/intermediate shaft pulley, or the cam pulley to give the Hall sensor something to read. You could even get a tone ring from an ABS System that's the same size as some other component in the timing system and try to use that.