Front bumper mounting issue

[Sinisterspeed]

Ok, i am finially trying to put the finishing parts on my truck. Here is the list of the issues that may be contributing to this problem.

Rear 2/3 of frame is from 71 GMC longhorn.
Front Frame horns from unknown year 4x4
Fenders are aftermarket
Core Support is from 71 GMC 2wd
Cab is 2wd 70 GMC
Hood aftermarket
frame has been welded smooth on the outside so no hole for outside bumper brackets. (Planning on drilling where ever it lines up to 4x4 brackts when i get some)

to do all the panel alignment the right front core support body mount was shimmed aprox 1/2 - 5/8"

now.. I have the inner bumper brackets from a parts truck i have here at the house. I tumbled and painted them. Mounted them to the upper holes of the new bumper.

The Issue.. Right side is almost covering the turn signal. left side about half of it. but all the brackets line up..

The obvious fix is cut the frame horns plate a flat edge and then move the bumper till it lines up with the body correctly and mount the frame horns back on to the plate welded to the frame.... Before I do this. I would rather "ask the pro's" is there something i am missing?