Big Bill allowed the Modified's Engines to have just about anything, but Sportsman Cars were only allowed anything you can't see (Limited Sportsman had to be under 300 ci).Modified Engines had aluminum heads, multiple carbs unlimited CIs, aluminum flywheels. The only rule was the engines must be of the same manufacture and remain in the same position. Quick Change rear ends were allowed.The Sportsman Cars that were flat head Fords (most were) outward appearance had to remain the same. We used to try to find Canadian Blocks (thicker cylinder walls) Denver Heads (41T), port andpolish between valve and cylinder. Gear Ratios were a big problem. If I remember correctly the standard Ford was 9/34,39 Mercs had 9/37 and hardest to find was 1940 Sedan Delivery 9/41. Heavy duty spindles were made out of 1937 thru 1939 3/4 ton rear ends. They were very hard to find. To check for aluminum flywheels they would pull your starter ( my car owner put our steel flywheel on a lathe and ground it so thin it would barely hold the ring gear). LOLAt 80 years old my memory is not the best, if I think of anything else I'll post it later.