Help on 340 combo

ok i was looking at things the 2 maine things i see is springs and t conv. i called ptc and thay said thay can loosen up the convertor. also what is the differance from stock over ss springs and what is the bigest tire u can stuff in the wheel wells. my tires with a 4.5 offset only has about 3/4 in clearance from the spring..will a 5000 stall change my top end rpm going through the lights ???