wheel offset for bagged 49 ford truck

I was wondering if someone out there could help me. Building a truck with my brother inlaw and i never worked on anything this old. We put a heidts mustang II front end on this truck with shockwave bages. It lays the running boards flat on the ground. i just need to know if someone knows what offset wheel we can run he is goig with 20s all the way around. We are trying to get this truck to a show in a few weeks. If anyone can help would greatly appriciate it thank u

I know on my 60 f100 with a heidts front end 56.5 I'm looking at 20x8.5 with a 4" backspace that's leaving room to be able to turn a bit layed out