Looking to get a set of Coddington wheels
2-18x7 4" backspace 2-20x9 5' backspace
Does anyone know if these would fit OK

 

I'd have to guess from your user name that you're talking about a '77 Camaro? If so, then there is a hump for the rear seat belt inside the rear wheelwell that might interfere with your BS on the rear. I'd also guess it's not really 5', but rather 5".
I use a pretty simple way to check clearances on my cars prior to springing for tires and wheels. I use a stick drilled to match two bolts on my hubs, and then measure out to where the outer rim edge would be. I clamp another stick on at that point that is set to the BS depth I am looking at. Then simply turn the axle, or hub to see if it comes close to anything.
You need to also look to see if there is clearance above that point for sidewall bulge, as tire height, and sidewall bulge can add interference to the equation. I personally think your choice of wheels, (regardless of tires) is pushing the limits on a 2nd gen Camaro, and will probably be too large, too wide, and too much BS.

 

Hello. At the rear, I mocked up Toyo Proxis 295/35/18 and looks very nice. The hump of the seat belt where he is saying will be an issue so you need to nicely hammer it inside. I have about 1 inch from the inside of the tyre to the inner of the wheel arch (near the aft frame rails) as well about 1/2 inch between the out side wall of the tyre and the inner of the wheel arch lip when the tyre is pressed inside the wheel arch. I have placed some pictures for your perusal. There is one photo which shows where the tyre comes touching the hump where 1971BB427 is saying. I still have not worked out the B/S but should be approx 4.75" to 5" if using stock axle width.

I am replacing my rear end and still not sure if I should bring it narrower to have some more lip!!!

 

I would assume both sizes would fit.

You are looking at a 7" rim with 4" of offset - sticking the rim out 3" and in 4"
I have an 8" rim with a 5" offset - same 3" out but 5" in

Rears you want a 9" rim with 5" backspace - 4" out, 5" in
Mine are 11" rears with 5.75" so 5.25" sticking out and 5.25" in

Provided your 77 is the same as my 70 front and rear you will have more clearance both inside of the tires and outside, unless i am looking at this all wrong?

 

You'll also find that most wheel suppliers will gain some on the front side spacing. A 8" wheel with 4" of BS, wont have 4" of front spacing. They usually measure backspacing to the inside lip of the rim where the bead seats. But tire companies measure to the outer rim outside edge on front spacing. Never made sense to me, as that would make an 8" rim with 4" of BS have 4.5" of front spacing.
In earlier years the wheel companies measured BS of FS to the furthest edge of the wheel lip. Like this: