There's two reasons for the slight "grind" shifting into reverse:
1. There is no syncro on the reverse side of the 1st/Rev slider/gear. Half of each gear set is solidly mounted to the main shaft and spins at engine speed. Syncros and the brake bands in all other gears allow the half of each gear set that is floating on the main shaft via bearings to monetarily match speeds so the syncro teeth can engage the sliders teeth mounted solid to the main shaft and thus locking the floating gears to the main shaft via the slider. When you depress the clutch, it allows the main shaft to UN-torque from the motor and spin freely so everything can match speeds via the brake bands and the syncros. I know that this can be a bit confusing to read without actually seeing it in action so you may want to read that again.
2. On the reverse gear it only has the straight cut teeth which must engage the reverse idler gear. These teeth on the Rev/1st slider have a cone profile on the edge to aid them in sliding into place and engaging the reverse idler gear.
With both of these conditions happening, it's easy to understand why if you push in the clutch and slide it immediately into Reverse, you will sometimes here a "grind" as the two gears mesh.
As for the 1st to 2nd shift, if you feel a slight "grind" as you shift into second, that is due to the brake band being worn on the 2nd gear syncro. Since this is worn, it can't match speeds as well causing the syncro teeth to jam into the slider teeth which in time rounds them down. So it leaves you with a worn 2nd gear syncro AND equally worn 2nd/3rd gear slider.
If you think you have a trans that is in need of rebuilding, I highly recommend picking up a spare trans on Craigslist for a couple hundred bucks. The internal parts are expensive(!!!) but with a spare trans you will likely have most everything you need. You can use 2nd-5th gear syncros on the second gear. They can also replace the 1st gear syncro provided you use the 2nd-5th gear brake bands on 1st. The 2nd/3rd slider is the same as the 4th/5th slider. The 901s are good boxes, but I would still count on having to replace the 1st/Rev slider (which is about a $550 part alone!!). This slider however is a literal marvel of machining and you'll soon see why it cost to much.
Hope this helps. . . Good Luck!!