One of the most complete online sites available for Locost chassis information is
The McSorley Home page has 3d drawings and much more.
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As to defining a "Standard" seven for a new autocross class, I don't know for sure.
I don't know what the goal is.
Do you want a class for un-modified factory built Caterham 7's? Or do you want class for limited modified seven like cars?
The SCCA Modified Advisory Committee (MAC) went through a long laborious process trying to come up with a set of rules that would bring parity to different types of cars, from unibody, to body on frame, and include the Lotus 7 type cars.
At least your task is limited to only Lotus 7, tube frame, and body type cars.
The MAC eventually defined what were competition producing items: Minimum weight, low center of gravity, wheel base, track, and horse power (engine size) were some of the items. The Lotus 7 was one of the obvious leaders in every one of those categories, so many of the minimum dimensions chosen for D/E mod were based on the Lotus 7. The minimum allowed floor pan dimensions are those of the Lotus 7. The floor pan minimums limit the length and width of the smallest car you can build (for D/E Mod).
Do you want to allow "in excess" cars?
Do you want weight breaks for different engine displacements?
Do you want to regonize the different performance potentials of OHV, OHC, DOHC engines?
What tires will be allowed, race, street, high perfromance DOT R tires?
If this is an SCCA class all cars will need to meet the minimum safety requirements of section 3.3 of the SCCA rule book. (roll bars, seat belts, etc)
Good luck, the SCCA has been working this problem (the Lotus 7) for ever. It all started when the Lotus 7 was approved as a "Production" car. Probably a bad idea.
Del Long