Auto trans shifting

I am buying a 1986 B350, 15 pass van. When I test drove it, it did not shift into third gear. Will you make a guess as to what might be wrong? small adjustment or tranny rebuild, or maybe just a fluid flush? Can I update this van with disc brakes and an overdrive or 4 spd trans?

possible problem with throttle valve linkage from engine to trans,,,,If it
has reverse ,,,,,should be able to get 3rd {same clutch in use},,,,,,linkage should be free moving and "pulled " tward engine.

Thanks for the feedback. I will probably pick up the van on Sat. I will pop the engine cover and check the throttle/ tranny linkage and waycth if it works right.

OK so I drove thre van home last Sat eve. I more checking, the trans does shift into third. I guess I am not used to a threed speed. From a stop light it shifted into second quickly and then about 25mph, i t shifted into third. I punched the accelerator on the freeway and it indeed kicked down a gear. SO, I am thinking I got a cherry deal. $1000 bucks for a 1986 15 passenger van with a 5.9L and 90,000mi. It has no dents and no chi :iceslolan ps in the windshield.