I am going to swap out the drum brakes on my 1970 Roadrunner with Disc brakes I want to go with 15 inch rims what suggestions for rim with and tire size? I am not going to race it I want to cruise and drive on winding roads what do you suggest.

Stock would have been 15x7. You can go wider than that in the rear. I think a 10" wide wheel will fit if you have the right offset/backspacing depth.

I agree , 15 x 7 all the way around , unless you want just a bit bigger in the rear. Then 15 x 8 will do you well.

Frank

I put 15x7 all around on my Roadrunner and recently put 15x7 front and 15x8 on rear of my Charger.... both look good. I like the 15x8 rear on Charger.....

On a '68, I've gone as wide as 15x8 steelies (4.5 or 4.25 BS depending on how you measure).
265/50R15 front, 295/50 rear, filled the openings nicely, didn't rub anywhere, actually plenty of room.
That 50-series sidewall looked a little too short to me, though nobody else seemed to notice.
Current '68 has 15x7 & 15x8 Magnums, 235/60 & 275/60, looks appropriate to me.
The 8s & 265/50s were a bit much parking with manual steering & Hemi.
The 7s & 235/60s not bad at all parking, manual steering & 383.
Member "mickm" here with a '70 seems to have tighter rear wheelhouses than my '68s.
15x10 with 5.5 BS & 295/65s is the most you can run on '68-'69 Plymouth B rear.

Here are 15x7 road wheels on my 70 Sport Satellite - 235/60/15 tires all around

The tire size that you go with will have some say in what size wheel you can use.

These are 15 x 8.5 Western Cyclone II wheels with Goodyear Rally GT tires on them, G-50's on the front and L-50's on the rear.

These pics are from 1982, before people started thinking that you could only put factory looking wheels on a car.