tire size ???

[thesledhead]

I know there are already a ton of tire size questions here already but here is a quick one. A friend of mine knew I was looking for a set of torque thrust's for my build and a buddy of his happens to have a set on his 55 chevy that he is going to tub out. Well all he know's is they are 17" front and 18" rear, they have new tires on them and they are cheap... very cheap. So we are driving out there this weekend and the question is what do I need to look out for when we get there? Such as offset and tire size? My truck has a 3/5 drop, a 73 cross member with stock 73 disc and a bone stock 69 drum rearend. Im not too worried about the front since I have some 17"s on it that I can measure off, but the rear???? I have no clue????