"...there's a very tight spot which seems to come around at regular intervals, maybe twice per firing order or so..."

The interval is important.

If this "tightness" occurs every revolution of the crank, then it's related to crankshaft stuff, such as tight bearings, piston-to-bore interference, etc.

If this "tightness" occurs every other revolution of the crank, then it's most likely related to camshaft stuff, such as interference in the timing gears, interference in the valve train, possible contact of valve-to-piston crown, distributor, and so on.

A little detective work is in order.

I've rebuilt six or eight B20 engines (essentially the same as the B18), and several with substantial mods, such as high compression, overbore, hi-lift cams (K and R grinds), double springs, and so on. I do not recall finding a "tightness" even on a fresh engine. I always prelubed everything with Amsoil differential oil first.
--
Don Foster (near Cape Cod, MA)