Not sure who you have to do your tuning but these are the codes that are normally turned off with a high stall converter. PO751 and PO757. The issue is because the computer see's slippage due to the converter stall being higher than what the factory spec's are. Sounds like your trans went into a limp mode condition (3rd gear starts) which really is not the transmissions fault. It is only doing what the computer is telling it to do.

I think the title of this thread should read I have a code on my FLT Billet transmission! Need help! Vince