How hard is to put on new rear Leaf Spring Mounts?

What do you mean by "new rear mounts"? Are you talking new weld on to the axle tube spring pads? This is not for the true layman. Not only do you have to figure width placement. But you have to consider pinion angle and both pads need to be on plane with each other. Then take in to account your either over zealous welding where you build up so much heat that you warp your tubes. Or your lack of proper welding penetration where the axle rips from the pad and twists in the u-bolts. But it can be and is done by home diyer's all the time.

As Clint says. A man has to know his limitations.