'69 Dodge R/T wheel size question-Mike from Mod Muscle?

[Nvrenuf]

I just got a '69 Dodge Coronet R/T convertible. I am still learning about the car. I really know nothing about MOPARS. I am keeping it stock, but would love to fit some big wheels/tires under it. I only want direct bolt on options. The rear wheel wells are HUGE! I would love 20" rear, and 18" front. WIth as wide as I can go in rear.

1. Can anyone here tell me what I can get away with. Specifics please.. (ie backspacing, etc.) pics would be awesome!
2. what is the bolt pattern on these cars. I think 5 x 4???

Mike S. from Modern Muscle, I know u can answer this.