sbc350 on delta88 diesel

[Delta88 Diesel]

Hey guys I put a sbc350 in my 78 delta88 that had a diesel and it runs great but im having some trouble with the transmission. It shifts to 2nd gear at 35mph and doesnt want to seem to shift into 3rd even after I hit 70mph when I have it on drive. However if I manually shift from low, to second, to drive it shifts great into 3rd gear. It still hits 2nd gear at 35mph and when I hit drive after that it shifts to 3rd gear. It also doesnt downshift when I leave it on drive, only if I manually downshift to second then low.
Any suggestions?
It should have the stock transmission, I havent been able to check the casting numbers, weather sucks.

 

[lilbowtie]

Kinda sounds like your not getting any vaccuum to the modulator or not enough. Should be hooked to intake vac and it should be
a black modulator. Diesel uses a pump.