I have a 1984 lg4 305 I may later put in a 350 sb. Which size or sizes is best for each displacement? Is it a difference of onlly RPM ranges or more so than that?

I see you asking all sorts of exhaust questions, and it looks like you're going to swap the entire system, from heads to street (headers, cat, & cat-back). This is the perfect thing to do, since it is exactly what the LG4 needs before you even try to do anything else to it to speed it up.

That said, you have the perfect oportunity to get rid of the single biggest restriction it has, which is the tiny little Y-pipe. DO NOT get any piece whatsoever that will bolt to existing LG4 systems!!!! Doing so will preserve that bottleneck. Get the entire system, but for some other car; either the whole L69 (HO) version, or the TPI one, for something like a 88 350. All of them will fit your car as an assembly, but none of the better ones will interchange at all with the LG4 stuff, because the LG4 has that drinking straw sized part. Again, DO NOT buy ANYTHING that the mfr says will fit LG4, except possibly the cat-back, since those are somewhat more universal.

Get the whole exhaust for whatever you decide on, complete. That's headers, cat, & cat-back. Get application-specific headers such as SLP, Dynomax, or Edelbrock, not "universal" ones; the direct-fit Catco cats from Summit or wherever are a good deal; and your choice of cat back from Hooker, SLP, Edelbrock, Dynomax, Blowmaster ,etc. etc. according to your taste and budget. You'll end up spending so much at the exhaust shop on making "universal" stuff fit, that the initial sticker price of the direct-fit stuff will actually be cheaper in the long run, because you can do it yourself.

There's not going to be a noticeable difference between 1 5/8" and 1 1/2" OD headers. There is some tuning effect to the diameter but it is somewhat overrated, especially in the case of a small-block chevy in an F-body.

Spend as much money as you can afford on good quality components. I have Flow Techs and they are terrible, several pinholes in the welds right out of the box. Inspect the headers for potential leaks before you drop any money on them.

I know the $$ SLP stainless headers are good, the Edelbrocks are generally good quality - can't say about others. There was no quality control taking place on the Flow Techs that I bought.

I have a set of FlowTechs from before they were bought by Holley and they're just fine.

Get some Hooker headers, Hooker cat-back.

Get some 1 5/8th headers unless you plan on building a decent 350 in the future. Right now I'm selling my Edelbrock headers 1 5/8th to get some 1 3/4 for my upcoming setup.