I have a a 1973 gmc with a 350 and a freshly rebuilt th400 but I’m having some problems with it. The transmission is extreamly sluggish. I doesn’t seem to engage until 1500 rpm and doesn’t was to go uphill out of a stop. It has a stock stall tourque converter behind a mild 350. About issue I’m having with the trans is that it is shifting way late(like 4000rpm) in every gear. It has an adjustable modulater that is all the way backed out so that it should shift with less vacuum. I shifts a little better after backing out the modulater screw but is still pretty bad. It doesn’t have a shift kit or anything like that and is a fresh rebuilt trans that’s tranny fluid is slightly above full. Any help is appreciated.

Yes I measured the vacuum and it’s somewere between 15-20psi. I thought that it might be the modulater and replaced it but I’m having the same problem. I energized the detent silinoid and made sure it wasn’t stuck and it engages and disengages.it still only shifts at wide open.