Slant 6 Help

Most likely, you have a 7 1/4 rear axle behind that /6. If you're dead set on swapping in a V8 then you're going to want change out the rear axle for something stronger like an 8 1/4 or 8 3/4. So with that in mind, swap out your rear axle now and while you're at it, get a lower ratio. (3.55 for example)
That's about the best you can do for now and still plan for the swap. Don't spend any money on the motor if you're sure you're not going to keep it.