When I had dedicated slicks and rims for my Camaro I simply screwed the slicks to the rims with big 1/4" hex head sheetmetal screws. I never had any backing plate inside, and I really don't know how you can install one? I divied my rim lip into 6 points inside and out, and just drilled holes before mounting the tires. Then once mounted I screwed the rims to the beads with the hex screws. I used screws the same length as the bead thickness, so the rim would hold them out enough to not penetrate through the bead.
That's about all I'v ever done or seen done.... no template's I remember. And how
do you get a backing plate inside a tire?
Mm, I don't think I would be concerned about using the previous holes.... there should be plenty of bead to just "split the difference" and drill new holes. As Vall mentioned, I just try to evenly space 5 holes around the rim, front and rear. After mounting the tires I would drill a small pilot hole in the bead, using a bit marked for depth with masking tape, and insert the screws. The best I remember, I used #12 or #14 x 1/2" sheet metal screws.
But since you have the Summit template I'd check to see if the hole pattern matched the tires and if so... no problem! And I totally understand the apprehension to drill holes in a brand new set of wheels........