It's possible to create a gear grinding problem on a downshift by using the wrong lubricant in the transmission. Some of the new synthetic gear lubes are too slippery to allow the blocker rings in the synchronizer to get a grip on the gears and match up the speeds during a downshift. Plain old 90 weight mineral oil is the preferred lube. Most farm stores sell a "Ford tractor" 90 weight lube that's the right stuff for a SM-319. There is no synchronization on 1st. gear on a SM-319. It uses a "low-reverse sliding gear". The later model Saginaw 3 speed used in the middle 1960's and later is a bolt-in swap to the bell housing for a 319, but it will probably need some modification to the shift linkage and possibly a different driveshaft to complete the conversion. The Saginaw has a synchro 1st. gear that can be engaged up to about 15-20 MPH or so.

Question- - - - -isn't the SM-318 the normally-installed 3 speed for your truck? A 319 has a Borg-Warner overdrive attached.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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