I think you've done your homework appropriately to this point.
Your current RPO L05 350 with swirl port heads have their place. If you want HIGH horsepower and high RPM, they are not the head for you. But they are a good truck head, and make great low end torque, which is what gets you moving, hence why GM used them in heavy trucks and full sized cars. They were also used in some F-bodies, but mostly as a low cost way into a V8. It gave them an intermediate step to offer the Camaro/Firebird buyer that the Mustang did not have. But I digress...
The RPO L31 5.7L Vortec will ultimately make more horsepower, and have more "headroom" for future performance growth.
Which is right, or best? Depends on your budget, your goals, and your patience in tuning. Understand the jump to the later 5.7L Vortec will require a better fuel pump most likely and expensive intake or an adapter for a carb intake, a chip or tuning system like EBL Flash, etc.
I would
seriously recommend you spend a few hours reading through the TBI section of the thirdgen.org forum. They have about the best TBI information you're going to find on the web today, including contributors Rbob, FAST355, Tuned Performance, Schurkey, and others.
If you thumb back far enough, you can find a LONG thread (lots of detail and questions/answers) from a fellow who made the jump from L03 TBI 5.0L to L30 Vortec 5.0L, and what some of the differences he ran into where (accessories, tuning, etc.).
TGO