56ChevyLS1

I just bought a '56 Chevy with a current generation 4.8L LS-1 and 4L60E auto transmission. While I can shift into Park, Neutral, Reverse, and the forward gears, the transmission ignores my column stick command to shift to a particular forward gear (1st, 2nd, 3rd) except OD. If I shift to 1st while stopped, for example, the transmission will still shift through all gears to 4th, in spite of where the column stick is pointing. BTW, the Ididit shift linkage looks good but I'm having it checked in a couple days just to make certain that's not the issue.

The car was "tuned" at a Corvette shop by the professional builder and I was told that the transmission did manually shift/downshift OKay at one time. I'm wondering whether the performance tuning erroneously changed a transmission parameter, leaving me only with automatic shifting through all gears? For example, the computer may now read all forward gear positions as "OD".

Is this possible and can someone point me to the parameter(s) to look at? I'm going to have to pay to have it corrected, and focusing on certain transmission parameters should make it easier? I'm guessing that the shift points might have been adjusted, but I don't really know what the "tune" did to the standard programming.

Thanks, I'm a newbie here and just trying to get my ride doing the right stuff.