Hooley, That's are great question and I'll try to answer it. First how many manufacturers are there of 28/29 Ford bodies are there or have been and do you think they were all the same? Of course they weren't and I believe the Austin was not as bad since so few were copied.
Using a previous drag car body may be a problem since they cut them up so much and many have had additional extensions from the original hood part line forward. When used in a G/F Roadster for the SCTA it needs to look like a duck and quake like one too.
Classglassperformance lists them, shows the extended cowl, center seating tonneau and the demensions. Speedway has the same one.
I will point out 2 occasions when fiberglass bodies were questioned and both were eventually illegal they were 26/27 fords used as modifieds: 1, had the cowl actually widened so the hood sides covered a 426 style hemi and every knew the 426 style was wider than the stock Ford body at the cowl line. Your not allowed to widen a body by the rules. 2. the builder took the body out of the box and set it on the floor and built a frame and car around it. He never noticed the bottom body line at the cowl was pinched in a few inches. The roadster was made illegal as the body demensions must remain stock. Both cars were fixed and continued to race.
I don't know what happens at any of the other land speed "pavement" events but bringing out ANY new roadster to a Bonneville or El Mirage meet will bring out the "inspector/critics" and if you have altered anything some one will find it. And if you do build one, any brand, for Gods sake DO NOT sand off any of the revel or trim to make it smooth; that will throw you out RIGHT NOW. I know of 3 that is happened too. I will have mine at El Mirage in November running there for the first time in 4 years baring the weather. I was loading for October when it was cancelled. Good Luck