For one thing the idea of building a 55 buick as a gasser is cooler than hell and the other it would be considered a rat rod by my book. My definition of a rat rod is anything built more for speed or function than style. Theyre usually built with some if not mostly all nostalgic parts and will resemble the early style rods. The only reason that flat black or primer is usually considered the only paint for rat rods is that in the old days a guys rod usually never got out of the primer stage since it was never done and anyways a good paint job cant make you go any faster. Though still having a good period style paint job would be acceptable. Were only trying to build rods in the style and spirit of the old rods not trying to build purposely cars that are rough or "beaters".
As for suspension the only way to go is either with a stock suspension, lifted or lowered, or with a straight axle. Sorry for being so wordy I just hate guys with billet laden cars that all have 350/350 combos, a 9 inch rear and mustang 2 frontends. Now they have their builder paint them flat black, throw a mexican blanket across the leather seats and they call it a rat rod when its nothing but a street rod.