Is There A Difference
 

Just wondering if there is a draw back in using a 350 trans with a ls motor. Also do 350 headers work on ls motors? If so any difference in performance?

No 4th gear, and the adapter for a TH350 to a LS engine negates the savings of the tranny. Gen1 small blocks have uneven exhaust ports as opposed to the evenly spaced ones of any LS block.

If you're trying to go modern on a tight budget, consider a mid-late 90s Vortec truck motor or an LT1. Your headers and a TH350 will work on the Vortec but if you're going a little modern, go with an overdrive and some fun gears and you'll never look back.

a 5.3 with tranny isnt that expensive as a take out package... get it with wiring and ecu