Sell the 350 and use the money to offset the cost of the rebuilt 4L60E. You will put a lot of money out just to run a turbo 350 plus you will lose overdrive. Cottmans quoted me $650 to rebuild mine as a carry-out. You will save many headaches if you just get the stock one rebuilt. A good salvage yard tranny may be found for less money and carry a warranty, also. I picked mine up for $400, so shop around. You will probably get SES warning lights due to no feedback from the trans about oil temp or other things it checks.