Aside from having to drill holes in the frame for fenders and running boards, you will want to make sure anything you mount, like shock brackets, etc. will clear the fenders later on. I would think your rear end tire to tire width would also be important so they will properly tuck under the rear fenders.......otherwise it will end up looking like some of the cars built in the 70's and 80's.

If your frame is fully boxed gaining access to the inside of the frame to put nuts on the new fender bolts might be an issue, but that could be planned for and built in.

Don