I currently have a 15x7(I assume 4"-4.5" back spacing) with a 235/60 tire and a 15x8(3.75" BS) with a 295/55 tire. would a 18x8(4.5" BS) and a 245/45 tire and 18x9.5(5.5"BS) 295/40 fit?
I think they will fit with the extra back spacing and some trimming. I currently have a set of 18"x 8" with 4.5 backspace on front and a 245/40/18" they fit fine. The taller tire may or may not be a issue. On back I have a 18" x 9.5 with 5.25 back spacing with a 275/40/18". The rear is too close to the wheel well opening for my liking. I could trim the wheel well or roll them and make them work. You can only go so far toward the qtr panel because the outer well will hit the tire. I plan on putting at least a 295 on back. I really think a 305 or possibly a 315 will fit on a 10" or 11" wheel with the right back spacing. I definitely have to have wheels made to make it right. The only thing stopping me is spending the $3500 for the wheels is making sure my measurements are good. I plan to do some fabricating to the rear wheel housings to give me maximum clearance before I strip and paint the car. I was surprised how much room I had between the tire and the frame with the 275.
I was under the impression the 68/69 cars had smaller openings. I have very similar sized wheels/tires and they fit fine on my 70.
They do have smaller openings and sit lower on the tire. When I have the 15 inch tires on I have to unbolt the rear coil over shocks to stuff the bigger sidewall in. I have to shim the wheel out with a 1/4" spacer to clear the rear disc caliper also. With the 18" wheels and tires I don't need to drop the shocks due to the lower profile of the tire and the disc brake caliper clears the rim without the shims. I have a old set of 15" Cragar street stars and they may fit without the spacer on a GM caliper but not with the baer brakes I have. I used the biggest rotor I could fit and use a 15" wheel. This way I could run a slick with bigger sidewall for the drag strip and a larger wheel for the streets to take advantage of the handling package the car has and the look I want. I actually bought the 18" wheel package off craigslist because it was a good deal and thought I might be able to use them. They are for sale and like brand new. I have $1050.00 invested them and will post them soon US mags U111 on Cooper tires.
As far as the tires themselves go, you're actually putting about 1/2" shorter on the rear, so if you can get your backspacing figured out, you'll be fine. Here's a tire size comparison calculator I like to use: https://tiresize.com/calculator/