If the 1st gear goes in smooth I would suspect a problem with the trans rather than the clutch. I know the nv3500 has a synchro on reverse that prevents it being engaged during forward motion. It can become a problem if the trans has been abused and not serviced properly.

Has it had a fluid change with the correct synchomesh?

Try going from 1st or 5th to reverse (Not while in motion) without letting off the clutch. That's kind of the old school way to get a smooth reverse shift, not sure if it works with more modern transmissions.

 

Yes I changed the fluid back when I first got it. Half a quart came out and the both plugs were filled with metal. I used AC DELCO SYNCHROMESH fluid.

I tried that trick earlier but it still grinded. I don't get how changing the clutch made it worst. The pushrod goes in and out of the slave cylinder, but idk if it's pushing or retracting just right. I read a bad slave or master cylinder will cause a grind from the tranny

 

Sounds like the tranny went south. Cant put metal back wear it belongs so a rebuilt, used or new are your only choices. My NV3500 holds 4 pints of lubricant so you have a real issue here, sorry to say.

 

You may want to drive it awhile before resorting to pulling the trans again. A little clutch wear may help, perhaps some debris got on the clutch/flywheel. Any air left in the hydraulic system may work out or into a position to be bled easier.

If the grind into reverse isn't too bad it may last a long time. I've had two nv4500s go 100,000 miles with 3rd gear grinds.

 

I must be lucky. My NV3500 shifts as smooth as silk. but then I do take very good care of my K-1500. I use the Royal Purple synchromesh fluid.