Why isn't my truck starting?!?!!

[Silverado6.6]

Hey guys, I hope I can get a quick reply on here....anyways, some of you may know that I've pulled out my tired 400 and dropped in a warmed up 355 in it's place....anyways, it's in, hooked up, bolted up, ready to go....other than, it won't start, it doesn't fire or turn over at all...I've got power to the dash, the seat belt buzzer works, my guages work, my radio works, but something isn't right w/ the starter....I've tried starting it in all the gears, and neutral and park, still nothing, someone said it may be the neutral safety switch....is this possible?? I ended up pulling out the tranny as well, but I don't recall any other wires asside from my kick down being attached to it....this is frustrating! I haven't been able to drive it for two months! and now that everything is ready, something probably minor is holding me up! damn it!!!! I hope someone may have a clue, thanks!