Rear weight bar and weights - Explain to me

[R420]

I have a 420 with snow pads and a Henry loader that I'm using to push some fill that has been sitting for a few years. The tracks spin if I try taking to big a bite. Should I add weight to the rear or will that put too much stress on the drive line? When were the rear weights typically used and how much was added? Any other info is appreciated. Thanks

[Lavoy]

It should have had, or did have weights at one time. Probably in the 1000lb. range.
Lavoy