Tim's right. Its not whether the gun can handle it, but whether you can handle it. Bench shooting is the worst, as heavy loads are downright brutal. I don't like shooting 405gr lead at 1700fps, so I really couldn't imagine them at 2000+fps! Then again, I guess they would be tolerable with the right rest, like one of the lead sleds.
But yes, the Handi can handle the Garrett loads. I've researched it since I bought my two .45-70's, and the pressure levels are (I believe) below 40k cup. If you look at the cartridges the Handi is available in, one is the .450 Marlin, which is factory loaded to 42-43k. There is no reason the Handi could handle the .450 Marlin at 40k and not the .45-70, so it will be safe with the Garrett loads (all though your shoulder may not).