Maximum Rear Weight for GT18?

I have a 78 GT18 with a loader currently mounted on it. Problem is, I apparently do not have sufficient weight on the rear when I load the loader (200-300 lbs), back tires tend to come off the ground and almost tipped twice. Currently have a 55 lb wheel weight on each rear wheel, can I put more weight on? Am I loading too much in bucket? Taking off the loader is not an option, have invested $1400 to get it operational. Any help/suggestions would be greatly appreciated.

I would recommend using dual wheel sets on the rear. Each tire could also be filled with anti-freeze to add more weight. An implement on the 3 point hitch can also give you a little more weight in the back. A scraper blade is a good addition.

I have purchased 2 lug tires and filled chloride for weight plus the 55 lb wheel weights giving me 120-130 lbs. per wheel which helped stabilized the back end when loading the bucket with dirt etc. My next plan is to replace the 4 ply front tires with a heavier 6 ply tires, so they flat out so much when I load the bucket, plus the original tires are 38 yrs old and starting to weather check. Power steering would be nice.

You need weight out past the rear end like on the 3 pt. hitch or build a weight box. I have a Kubota with a loader and guys always talk about how important it is to have weight on the 3pt. hitch to get the weight off the front axle and make the rear axle like the pivot point. I didn't think it was that big of a deal but I bought a 3pt. dirt scoop and was moving dirt with it and the bucket and wow what a difference it makes. Hard to explain but it just feels so much more balanced. If I am moving more then one or 2 loads of dirt I put the counter weight on. Plus it makes the tractor steer easier.

That is my next plan, to add a weight box/rear blade to my 3 point. Currently I'm missing the electric actuator, once I find one to mount and get the 3 point working, I'll add weight box/rear blade. Do have a plow and cultivator.