Not sure what rumor you're talking about. You can put an R200 in any Z, given that you have the correct mounting equipment, i.e. mustache bar and curved rear transverse link bar. This is covered extensively. A stock flat-top L28 block with a stock E31 will have compression in the low 10's and will definitely require high octane fuel. Shave the head and bore the block, and that number goes up. FWIW, the reason that you have trouble with the vac advance of the ZX distributor is because it's designed for an engine with EGR. An engine with EGR needs more advance. Too much advance isn't a good thing and you'll be better off either disconnecting it or running a different vac advance module. The late L28 (F54) came with flat-top pistons, it was the early L28 (N42) that came with dished pistons. One caveat is that turbo blocks (F54) came with dished pistons. You absolutely do not "need" to put in larger valves into an E31 that's going on top of an L28. It gets "recommended" a lot but is not a necessity. This is not a race engine build. Keep in mind that bigger valves, especially on a head with a small combustion chamber, will be more shrouded than smaller ones. This has potential for negative effects unless dealt with. A good build goes much further than a parts list. The only specification you mentioned is "it's just for street". In that case, I'd recommend a stock L24/26/28 with triples, open exhaust, 3.9 (or numerically higher) R200 and late ZX 5-speed. The diff and trans wake up the car way more than a mild engine build. Pertinent information is plentiful and oft discussed, spend at least a few months on reading and planning to make sure you know what you really want. For example, DatsunZgarage.com can get you started with the basics, but don't take everything you read as gospel.