Saginaw 3 speed problem
 

Installed rebuilt Saginaw 3 speed. Trans will not down shift from second to first while moving. Trying to downshift to first results in grinding noise. However, if car is at a dead stop, with engine running, if I shift into reverse and then into first gear it engages easily with no grinding. If I shut the engine off it will shift easily into first.
Any ideas ?

It is a non synchronized first gear trans. Or the first gear sychro is messed up.

Saginaw shifting problem.
 

I was afraid you were going to say something like that. But thanks for responding. Anyone interested in buying a 64 convert, 283, new top, tires, rear bumper,flywheel, pressure plate,clutch disc, radiator. Located in NC.

sounds like there is not enough airgap or clutch freeplay..
based on ''if I shift into reverse and then into first gear it engages easily with no grinding. If I shut the engine off it will shift easily into first.'':)

This. Pretty sure that the pre 70' transmissions were not fully synchronized.

found this on another site similar issue??
SAGINAW 1966-81 Cars, 1966-87 Light Duty Trucks:
Parts List 92Cast iron case and extension housing with 7-bolt side cover.
Fully synchronized with 30 tooth synchro rings.
1966-69 units may have electric overdrive.
It has a four step cluster and all the gears are helical cut.

how many bolts on the side cover?? 4 or 7-bolts
4-bolts are the non-sync 1st gear models it appears[earlier than 65-model year]...

If you recently changed the clutch and pressure plate, did you wipe the pressure plate down with thinner to get the oil off it?

I agree with the above, I'd look at the clutch first..