i know this has been answered before, but after a BUNCH of crawling around underneath my car and doing a ton of measuring, my old noggin' is still hurtin' from trying to think this out.
anyway, here goes............
as my car sits now, it is roughly a "+2" ride height with the rear springs.
it has hoosier 29x12.50 tires on factory 8" ramcharger wheels, that have 4"-4 1/4/4 1/2" back spacing, depending on how a guy measures the wheel.
i have a good 1 1/2-1 3/4" space between the springs and the side wall of the tires, and [roughly] 2 1/2" between the sidewalls and the 1/4 panel lip.
with that said, i want to change the wheels to torq-thrust style, and i need to figure out if i can go to a 10" wheel on the rear, or stick to the 8 1/2" wheel offered.
i want to re-use the tires, and they have an 11 5/8" section width mounted on the ramcharger wheels. i measured the width using a coat hanger sized wire, making the width between points so as to not deflect any as i slid the points across the sidewall bulge. if i mount these tires on 10" wheels, how much will the section width grow ? these tires have a 10 1/4" tread width.
what wheel can i fit using the tires i now have, width wise and back spacing ?
i seem to remember something about 4"-5" back spacing on a 10" wide wheel, however, i can't remember what size tire was being used.
TIA to all that can help !

This is an 18 x 10 aluminum wheel that has an outside width of 11 1/8". I have 4 3/4" back spacing and there is approximately 3/4" from the sidewall to the leaf springs. 5" of backspacing will work but the tire may make contact with the leafs if the sidewall is tall.

Best bet is to find someone who has already done what you want to do if possible. Finding out that size of tire section width on a 10" rim is a good place to start and then you can work on backspacing needs.

Get a large piece of paper and draw the wheel and backspacing + tire clearances to the car body and leaf springs ect out to 1:1 scale as a cross cut section view of what you have now and tape it to a window like a light table. After that you can draw up and overlay the other wheel/tire and backspacing setups you are looking at over top of the original one to get an idea of how the new setup should work.

I did this method two times on two different vehicles dealing with different tires and backspacing. It helped me see what was going to happen ahead of time as once you mount the tires to the rims you own them. If you have access to a cad software you can do this same thing on a computer fwiw.

Go with a 5-1/2" back space with a 10" wheel.

This tool- while costing $100 could save you hundreds or more. Its a cost savings you ALREADY have the tires you plan to run.
https://www.speedwaymotors.com/Wheel-Fitment-Tool-for-Tire-Fit-Testing-Size-Measuring-Mockup,485869.html?utm_medium=CSEGoogle&utm_source=CSE&utm_campaign=CSEGOOGLE&gclid=EAIaIQobChMI1I3Uv9Hg_QIVMTizAB051Q5FEAQYASABEgJRW_D_BwE

Or you can do it for free. This is how I sorted out what sized tires to run on my 51.

http://www.yourolddad.com/tires

have you done this ?
somewhere, the 5" figure sticks in my mind.......
here is my car now with the 29-12.50's mounted on the ramcharger wheels.
i thought i had a picture of the tire/spring/outer quarter placement, but i guess i don't [or i can't find it]

5 1/2" will not clear but go right ahead and try it.....
Take a straight edge laid against the drum and measure back to the leaf springs. 5 1/2" will likely be as close as 1/4" from the spring. The tire sidewall bulges out from there and will surely make contact.

over the years, i have had quite a few tire/wheel combinations on this car, but of course i didn't pay attention on any measurements.
at one time, i had some 11.00x[??] slicks mounted on 10" western knock-offs of the cragar ss wheel, and they fit and didn't rub, but as to the back space of those wheels, who knows ?
as i have said, i have roughly 1 3/4" [or so] from the sidewall to the springs with the back spacing of the ramcharger wheels, so i could probably pick up some there. that would/could allow me an inch, which would be around a 5" back space if the ramcharger wheels are a 4"-4 1/4" back space, and still have around 3/4" of sidewall clearance to the spring.
the best way of course, is to take the wheel[s] off and just measure everything, but i'm lazy, [] and thought i would ask if anyone had used a combination i am contemplating.

Straight edged clamped to the brake drum, or a bare wheel mounted to the drum, and measure from there.
Sidewall bulge will be the same on both sides and that spec is usually published along with actual tread width.
Measure both sides of the car, they can differ by 1/4" (especially A-bodies).
The good news is that Chargers & Coronets are more forgiving than Plymouths as the Dodges have larger openings.
The usual wheel BS is 5.5" on a 15x10 with large tires, but 5.0" might be fine on that car.
I've run 1/4" wheel spacers on my drag car with no issues - longer wheel studs, of course - if fine-tuning is needed.
IIRC, the wider/deeper Americans use a 3.75" BS...so measure twice, order once, ya know...

I'm a bit simple-minded, but to me the math is easy.
You say you currently have "a good 1 1/2-1 3/4" space between the springs and the side wall of the tires, and [roughly] 2 1/2" between the sidewalls and the 1/4 panel lip."
First get a better measurement to the leaf spring, but then you can add +/- 1" of backspace to what you currently have. All you gotta do is measure how much backspace your current wheels have.

it's measure 4 times, cut twice, and it's still too short.............

i know all about both sides being different. been around these cars for what seems like forever.
i really don't want to have special wheels made, but i may have to pry open my moldy wallet and do so.
i have noticed, however, that if you have a GM 4 3/4 or 5" bolt pattern, those wheels are often available with a 4,5, and 6" [] back space.
i'll figure something out. will probably take me awhile.
no wonder i never get anything done.

you are correct Mike.
i need to quit being so lazy and just "GIT 'ER DUN !"

The section width of the tires you have now is going to change when you mount them on a wider wheel...

I have 9.5" rear wheels with a 5.5" backspace with a 275/60/17" tire on them...

They are just almost too wide as some off camber entrances to different locations they will rub a little on the inside...

A pair of 1/4" spacers fixed that...

The tires come out right at 30" tall...

With a taller tire the section width gets wider on a wider wheel more so than those that Kern is running on his car...

I'd think a wheel with a 1" deeper offset than you have should be good as the section width may only gain .5 to .625...