Questions for Ls1 swap guys

[GMCRob]

I just finished up a LS1 swap in a 71 chevy truck and everything was working perfect exept my tranny wouldnt shift if you got on it hard, it would shift fine driving normal. well at the time my computer wasnt tuned yet so i figured it was that, Then the tranny went out so i put another one in it and took my computer to a guy and had my gear ratio and tire size changed and took all of the stuff i didnt need out. Well it is still doing the same thing, I am afraid of burning another tranny up. What else could it be, some kinda sensor or something , my computer is not throwing any codes, and the guy that did my computer does all the cars around here so i know hes good. Any kinda help would be great Thanks.