So I'm looking at buying this super cheap '86 Camaro as a project car and the guy says that the trans wont shift to fourth gear. It's an auto with just over 50k miles, any ideas as to what could be the problem? On a related note, at what point/speed should it - on average - shift to fourth gear?

 

4th gear would depend on load, The issue could be a few different thing but when buying a car with ANY transmission problems assume a rebuild or a junk yard trans.

This could be a the fourth gear seal is blown but you would not tear the transmission down and replace one seal. You end up with a complete rebuild.

 

Ok thanks, that's what I was thinking would be the case but wanted to check, I want to make it a manual anyway.

 

If manual is your thing, then you might look for a manual equipped Camaro?
Don't get me wrong and I don't mean to rain on your parade, but, a lot of work and changeover needed.
Flywheel on back, electrical items, such as Clutch Safety switch, back-up lights, clutch pedal with slave cylinder, crossmember maybe, with driveshaft.

But, as I say, 'Experience is the best teacher.'
Good luck with the project.