Reply to "1st and 3rd grind"

Have you checked your motor mounts? If they are old then they can sag on one side, which will put the shift linkage out of alignment and cause the length of the thow to be shorter into 1st and 3rd and longer inot 1st and 4th. Giving good solid shifts into 2nd and 4th and weak 'not quite there' shifts into 1st and 3rd. Seems like a plausible cause sine if the syncros ar worn frm speed shifting, then the speed shifting is usually done going into 2nd too, yet it is OK. I recall a section in the manuals about the measurements at the shift gate, for a properly adjusted shift linkage. That would change if the motor is shifting due to motor mounts being loose or worn. Is the shifting buttery smooth, or feel like it has friction in it? That's another clue things are out of alignment with the linkage, possibly due to motor mounts or the trunion bearing twisting from usage. Seems like a few more things to check 1st and good reasons why it may not be the synchros. If it was the syncros, then 2nd gear synchro is typically the 1st synchro to go bad, and you didn't mention any problem with 2nd gear syncro existed. Common sense approach says it's not the syncros since 2nd is OK. Like the other guys say, check to make sure clutch is fully disengaging, but that doesn't explain 2nd and 4th OK. I strongly suspect motor mounts, and they are easy enough to inspect and replace, and not nearly as costly. Avoid poly motor mounts, they melt.