Rim Help/Backspacing Info Needed

[Fatherflash]

Need help/advice from the team.

I have a 1972 chevy 1/2 ton long bed which will eventually be lowered, probably bagged with Porterbuilt suspension. Suspension is presently stock with original rear end installed.

I have an opportunity to purchase a set of used Boss 20 inch 304 rims with 255/35ZR20 97W nitto tires. I understand I should consider 40 series tires however, these are mounted on the rims and in excellent, almost new condition. I will likely swap tires eventually.

Rims are in almost new condition with damage to rims evident.

Seller is reporting backspacing on the rims is 5.5 to 6 inches. Rim width is either 8.5 or 9.5 inches.

Will these rims and tires fit on my truck?

Picture of rim with mounted tire is attached.