I have a 67 GTX 18 spline 4spd and after putting the car in neutral at a stop light it is hard to put into 1st gear, I have had the trans gone through and with new synchros/brgs from the kit from Brewers as well as put in the newer style roller bearing pilot brg and am using the Amsoil 75/90 synthetic gear oil that is GL4, the trans will down shift fine in all gears, its only when put into neutral at a light and than back into gear that it is real stubborn to the point that sometimes not sure it will even go into 1st or 2nd gear.
Any ideas would be appreciated. Thanks.

I'm not a 4 speed guru by any means, but if I were having that problem, the first thing I'd do is check the air gap in the clutch.

If that checked out good, I'd dump the trans fluid and try some Pennzoil synchro-mesh gear oil.

Don't think changing gear oil will do anything, the gears are not moving.

Double check the shifter alignment. I had a wore out shifter on my 300 that would get the shift handle stuck between the forks on the shift mechanism, it first showed up as difficult to put into first at a stop.

Who did the rebuild/ There are 2 different synchros used in different year 18 spline transmissions. Possibly might have used the wrong ones? Or you have a broken synchro ring.

Check the clutch adjustment, if you can have someone use a feeler to stick in between the clutch disc and flywheel or pressue plate with the pedal depressed like you drive it see how much air gap you get and let us know
Mopars are famous for dragging clutch disc
Which makes them hard to shift and put into gear
Good luck, you CAN fix it

I remember this was a common thing back in the day.. Seems like it was easier to put into second to stop the gears, then move up into 1st.

I would say based on your description that the most likely candidate is,

Improperly adjusted clutch creating drag. An indicator of this is how close off the floor does the the car start to pull the motor down and move ??

Pilot bearing / input shaft issue, again creating drag.

Mis-alignment of the bell housing, again creating drag.

An internal Trans issue.

keep us posted

The syno rings and hubs were all changed to the 70 style a few years a go.
I believe there is air gap as I had checked that with a helper when I was under the car, also when the car is running if you depress the clutch by hand it takes a while till you can feel the release brg starting to contact,
I will give it a bit more air gap and try to see if that helps.
Thanks.

I think that you will find that if you have between .060 and .100 air gap between the throw out bearing and pressure plate fingers with the clutch pedal up all the way the clutch will work way better

I changed to Amsoil sysnthetic gear oil in my 4 speed and it did not shift correctly. Honestly it's been so long ago and I left it in for a short time only, so I forgot exactly what my symptoms were. I think it was just hard shifting. I called Jamie Passom and he told me the old 833 needed regular gear oil the make the synchros work properly. He told the Amsoil was too slippery.

Synchros do not work when at a stop and shifting into any gear.

WHAT ??? The job of the synchronizer is to slow down the gear to ease engagement. The brass ring grips the cone on the gear and slows it down before the teeth engage. I would look into the shifter adjustment first, then the dis-engagement of the clutch.
Excessive bellhousing runout can also cause low speed engagement issues. You may need to change the fluid to a more syncro friendly fluid. I just use regular old dino gear oil and stay away from synthetics because they are just too slippery for the syncro rings grab the gears.
If this happened all of the sudden I would look into some mechanical issues like shifter issues or something in the clutch linkage.
Gus

Go back and reread what I wrote then tell us what gear needs slowing down when at a stop. Hint, NONE.

All I can tell you is I bought an NOS NP 833 installed it with regular gear oil and ran it for about 10,000 miles. I changed to the Amsoil and it shifted poorly. I changed back to gear oil and all is still well.

Do you recall as to exactly what oil you ended up using as I would definitely give it a try, was it just regular of the shelf 80/90 gear oil??? Thanks

The op says he's in neutral, then can't get it back into gear.

If the trans is in neutral, and the clutch is engaged, everything is turning.
When you push the clutch pedal in, there is usually still some amount of drag helping to keep everything spinning, and when you go to put it into gear, the synchro has the job of stopping the rotation and aligning the teeth so it will slide into gear.

Surely you've heard someone grinding a gear to get it engaged........ They don't grind when they're not turning.

I still say a fluid change would help, and the Pennzoil synchro-mesh was recommended to me by a guy who does a lot of transmissions.

It's not that expensive and it's easy to do.

I have also heard that slippery [ synthetic ] gear lube doesn't work well in older transmissions .

NAPA sells a GL-4 rated gear oil. Just make sure its not the GL-5 rated stuff. I also head that Pennzoil Syncromesh works as well.

I think I bought the Pennzoil at AutoZone or Advance.

Were the shift forks replaced?

Does it grind when it finally goes into gear?