1928-29 vs. 1930-31 Ford Roadster bodies in LSR

[Want1937hd@aol.com]

This question came up on another board I read, why do more 28-29 bodied cars 
run at LSR events? A current book on Vintage Dry Lakes races has many photos 
of 28-29's but not one 30-31. I do know that a 28-29 firewall and cowl section 
can be unbolted and this fact has something to do with the SCTA rule on where 
the body/firewall starts or ends. Could someone please explain this? Happy New 
Year! Bob in connecticut