Just bought my first square body. 1979 k20. The guy took out the manual trans and put a TH400 in. The issue I'm having is when you give it gas the truck crawls forward give it more gas and it still crawls as of it was just ideling till it shifts. When it shifts to second it will pick up speed like normal. I replaced the fluid, filter, vac modual, checked the gov. The trucks got a base 350 gm crate, lifted 6" on 35x12.5r15s, np208. The trans tag says FWA 90-FWA. I'm thinking either the trans is fried or the TC is for a big block.