the tranny on my car has supposedly been rebuilt, and it feels great when shifting, especially the shifter itself. the only problem i've run into is if the car is rolling forward and i try to put it in first, it grinds real bad.

the car was sold to me w a bunch of junk in it, and it looks like a clutch slave cylinder is amongst the parts. is that what would cause this?

 

Mines does that also. Don't ever try to force the car to go in first gear as you are probably doing. 1st gear is just for a small amount of velocity after that 2nd gear is fine.

Remember the tranny doesn't wanna go in first anywhere from 5mph+

 

Only grinds going into 1st, then the syncro is likely either worn, or not installed correctly.
If it was the clutch causing the issue, it would likely grind on all shifts, not just one gear.

 

If it goes into 1st, without grinding, when stopped, it's the driver, not the tranny. You need to either come to a full stop, or learn to rev match. These trannys do not handle 1st gear engagements, while rolling, well at all.

 

Could also be a weak hydraulic system. You might want to try bleeding the system and see if that helps pull the clutch away from the flywheel. But yeah, you have to be damn near stopped before shifting down into 1st gear.