danielsc101970

So I recently bought a 5.3 ls enguine with a 4l60e and put it on my 1970 c10 but I can’t get it to run right it starts off in 3rd gear I have to manually shift it to first and shift it to drive I feel it shift to second and third and then it shift down to second or if I put it in overdrive I have to floor it for it to go into third and forth but when i floor it takes off here is a short video

bbond105

It sounds as if you don't have it wired correctly. You can check out this website form wiring info. LT1 Swap
If you have a scanner that can read transmission data, that will go a longs way in helping to find out what is going on.