Marty Flood here. I will address your questions and concerns here and hopefully they will help guide you. The USRA rules on the site will be helpful to lead you how to properly go about designing aircraft for a specific class and submitting those plans for pre approval. I will give you your West Coast Reps email ( I know you want to talk directly to a person, but out of respect for their privacy i will let them give you there phone number). I suggest you make your first email to him. he will talk with you and lead you thru the simple steps for what you desire.
As far as Warbird class nd Sundowner class, They are pretty much extinct. We had 4 entries on one and 2 in the other so to make it possible, for those with those planes, to be able to race we introduced the sportsman class which welcomed them as well as other old non competetive type former racers the chance to once again compete. With that said, you would be designing a plane (warbird I presume) for the sportsman class. And as Tim previously stated all it needs is to be 80" min wing span and race in a bracket class of 2:00 min and 2:30 Max. If you want to design a scale one that meets the unimited class rules then submit those drawings to your West Coast rep Chuck at this email: firstname.lastname@example.org
Give him your number and he will return your call. I look forward to seeing what you create.