Bolt circle adapter/ backspace/offset questions

Adaptors are perfectly safe if used correctly and made out of a decent billet aluminum, no cast adaptors. They've been used on race cars, off road rigs, you name it. As long as there's enough metal to support the studs it'll be just fine if you keep everything torqued. It's not magic, it's physics.

The decent adaptors are usually ~1" thick, so, you'll need to add 1" to whatever backspace would normally work.

On that note, what rear axle is in the car? Because 8 3/4's and 7.25's are different widths, and that car could have either one originally.