transmission or vaccume trouble

[78 Beauty]

I have a 78 Monte w/a 305 and noticed that it doesn't want to shift at 35 into last gear. What should I be looking for?

 

[Evan11]

Do you have the th-350 trans? Could be the modulator. $10-12 part

 

[GT_80]

Do you have the th-350 trans? Could be the modulator. $10-12 part

that or the detent cable too tight, or something internal wrong. Start with the easy stuff- does it shift down into first and sencond and third manually? Have you tried manually shifting to see if it goes into third? Could be the linkage is out of adjustment too