So for my 91 RS LS1 swap, im debating if it'll be easier to use my existing gas tank or swap in a plastic 4th gen tank/pump assembly. I know there's threads on this but Im still unsure. If someone could throw some advice my way (specific) I'd appreciate it.

 

In my opinion, dropping the tank and putting a better fuel pump in (like TPI or Walbro 255) is much easier than swapping in a 4th gen LS1 tank.

You will need to do something about fuel pressure regulation. The choices are a Vette filter/regulator (which will require some plumbing work), or a return-type reg up in the engine compartment (which will require some plumbing work, but probably not as hard as the Vette filter/regulator). The return-type pressure reg up in the engine compartment has advantages, in my opinion, and you can use the TBI filter as-is.

I've done all three, by the way - 4th gen tank, Vette filter/regulator, and return-type reg (although the return-type reg wasn't in a 3rd gen).

 

If it helps at allI'm in the middle of swapping in a 5.3 into my '87 IROC and am going to go with the Granatelli GM-722-340 since it is a direct replacement and seems like the best value that I've found. http://www.summitracing.com/parts/grn-gm722-340?seid=srese1&gclid=CNCqk4-tuMQCFQqFaQodTksACA

As for the regulator I'm going to go with an adjustable one under the hood and use the factory return lines, I just haven't decided on a brand yet.

 

Im just finishing up my swap. I went with a walbro 255 and the vetted filter/regulator. I think I'd just avoid swapping in the plastic tank.