The rat rod approach to building a 55 Buick would result in a beater not a rat rod. A gasser is more race car than street rod. They weren't originally intended for street use. If you put a gasser on the street I guess it could be considered a street rod but it takes too much work to build a gasser for it ever to be considered a rat.
I'm glad you didn't ask if you should put a straight axle under your car because most of the guys on here would say not to do it. But you seem to know what you're doing and if you have the skills, tools and freinds to build a gasser I think a 55 Buick is a great choice.
The straight axle and engine setback are only part of the gasser look. You need to move the rear axle and wheel opening forward 10 to 12 inches and have wheelie bars hanging down in the back.
AL