I was out cruising tonight and I did a 2nd to 3rd gear pull then let off. I shift into 4th and it wouldnt engage. It felt like it went into gear, but it felt lose and when i gave it a little throttle it would just grind. Almost like its not going in all the way or at all. Any idea? I hope i dont need a rebuild. I have had zero issues with my transmission and this is the first time. Its a jacks stage 1 tranny. Im not sure it it matters, but i just recently installed a weighted brass shift knob. Maybe I wore out the shifter bushings and its not engaging properly now.

 

I had almost the same problem. When I was trying to put in 4th gear it was grinding. I had also aftermarket synchro gearbox and aftermarket clutch and I faced that issue when it was new. I contacted gearbox manufacturer and I get the below answer.
" Many clutches do not release well or not adjusted enough to release fully . People tend to blame the synchro but it is normally (especially on newer transmissions) the result of a clutch that is not fully releasing and a small amount of drag over powers and over heats the synchro causing it to fail."
In order to find out if this is the case he suggested to do the below test
"If you put your car on a vehicle lift and put it in 1st gear, raise rpm to 6000-7000 rpm while pushing in the clutch I think you will see the tires will move.Make sure all 4 wheels are off the ground when doing that test. The pedal should engage the clutch near the top of the travel."
Personally I didn't manage to perform that test, so I am not sure if my clutch was adjusted properly. I replaced synchro and that fixed.
I hope that will help to resolve your issue.