Looking for more pep off the line

Might be on to something with the transmission, when I first put my 340 in my old car it was with a 727 the first test drive I was very disappointed with the take off, but it went really good down the road. It didn’t take long to find out the transmission was starting in second gear and only shifting once into third. Took the valve body out cleaned it and double checked everything. Don’t remember what I had in the wrong place. But it shure took off when I put it back together