I had rust issues on my rear spring bracket so I bit the bullet and got a kit from Mancini to relocate the leaf springs.

I'm set to drill my front spring holes on the boxes. Should I just drill stock spring location hole or would it be better to drill 2 sets of holes in each(higher & lower)?

This can needs all the help it can get hooking up.

I'd drill two sets - one stock, one an inch higher. Make sure the spring eye will actually fit in the box at one inch higher. That will depend on how high up in the frame rail you weld the box. The one inch higher setting will cause the spring to hit the tire harder, much like moving the front eye of a ladder bar up to the top hole. Also, make sure your rear spring shackle is installed so the shackle bracket can let the chassis rise - top spring bolt in front of the rear one at ride height.
Along with the spring change, make sure you have enough shock travel. Moving the spring in shortens the installed length of the shock, which is a good thing for travel.
Doesn't 73 and up have the rubber isolators between the leafs and the shock mount and u bolt bracket, and maybe rubber body to frame bushings? I assume you are not reinstalling those. Any rubber bushings between the body and such should be replaced with solid ones.

I had switched to the 71 springs and a 72 B Body housing shortly after getting the car.