69 rs/ss w/
18x9.5 5.5 bs and 275/35-18 tires
18x8 4.5 bs and 245/40-18 tires
just trying to get some ideas

I would say yes (it'll be close) as I have almost the exact same thing in 17's on my 69; 17X8 w/4.75" bs and 17X9.5 w/5.75" bs with 275/40-17's and 245/45-17's. I have rear discs so the 5.75" bs works. With drums I'd run the 5.5" you mentioned.

Jody