You have everything you need for a top swap?
-Heads
-LIM
-Blower
-Tensioner
-GTP PCM
I'd suggest to do headers with the top swap. Its the perfect time to install them. Grab some Speed Daddy's off eBay for $220. You'll be on a high compression block, with stock manifolds that breathe no better than coffee straws, and a blower that makes a bunch of heat. It's asking for trouble. In that case, headers should be necessity, especially when they can be had for so cheap.
I don't know what your budget is, but if I was going to top swap, I'd do the majority of mods I wanted to do now, such as Springs/rockers/Gen V/Northstar throttle body.