Rally Rim questions for my 68 Dart GT

Hey Everyone
Thanks for all the info you have given me but I am still a little confused ..........What I am trying to accomplish is to have a little wider tire on the rear of my 68 dart gt.......... right now I have P205/70R14 tires on a set of Rally 14" SBP rims which I assume have the factory back spacing. On all the tire charts I see, I can't run a set of P225/70R14 tires on a 5.5 inch rim as it is to narrow. As mentioned above in other posts - I have found two company's that sell a 6 inch wide Rally rimw/ SBP which would work with the 225/70R14 tire but I don't know what back space I should have - I currently have from the inside of the current tire - 205/70r14 about 1 1/2 inches to the leaf spring and about 1 3/4 inch from the sidewall to the inner 1/4 lip on the wheelwell - I can't cut the lip on the 1/4 panel because I have chrome wheel opening moldings to put on the car. My question is:

When a rim has a higher number on the back spacing, does that mean the rim will stick out closer to the 1/4 panel lip OR will it stick out closer to the leaf spring?

This is not a race car - simply a cruse night car and tooling around town car - no plans to race as it has a 318 V8 !

Thank you very much everyone !

Mike