Your asking a lot from your friend for a ride home in my opinion. Having adapted several loaders to tractors I can tell you just adapting one involves quite a bit of work. Building one is a major project. Your far better of finding a used one.
As far as the grapple goes they add a lot of weight to the front of the tractor. Hate to sound like a broken record but the front spindles and axle were very likely never intended to carry the loads your planning to put on them. If you think about it if you load the loader until the rear of the tractor gets light you now have almost the total weight of the tractor, loader, and your load on the front axle. Trust me you'll find yourself in this position far too often. Larger tractors can handle those loads GT's not so much so. I have my relief valves set for around 400 lbs lift. Even that is a large load for a small tractor.
Good luck with your project.
I would pay him well, after complete maybe a good sized bottle of rye too
I’m aware of the points you raised with loads shifting, had old boss’s 100 hp loader tractor lift the rear end off the ground a few times, I appreciate your concerns