The dimensions that would be really nice to know before you start the frame:
1. The rear axle width. Turns out that the +2 (see below) is about perfect for an S10 solid axle. So if you're going solid rear, that rear axle kinda set the tone of the frame. But if you are going IRS then I guess you can make those half shafts any width you want them, just need to figure on building the rear of the car differently (see below on the Gibbs/Haynes book).
2. The engine. My +2 build would hold about anything stock SBC and smaller I think, both in terms of size and strength. But if you're one of the crazed fools who have a spare BBC or 440 Six-Pack you're gonna use, you might also want to consider a bit bigger tubes and bigger space. And life insurance, but I digress.
3. Butt space. If you or a significant other who will be your passenger is one biscuit shy of being called "Fluffy", or you want something other than the bench seats in the book (and the original Seven!) then you might consider a wider chassis. You might set up a little space with boxes, etc to simulate the narrow bench seats of the Book cockpit to see if this is what you want. The original Seven had 15" of butt space for the driver, the Book I think has 17", etc.
Totally unsolicited advice --
1. The Ron Champion "Cheap Sports Car" book was a great start. However, lots of builders figured out that there are some measurement issues, editing errors, etc. And then the really smart guys figured out how to make the frame stiffer, bigger (to fit American-sized butts and modern engines), etc. Most of these were incorporated into the Chris Gibbs book, "Build Your Own Sports Car: On a Budget" aka the "Haynes" book. Sadly it too is out of print, but there are used copies around, perhaps a nice friend can help you, etc.
Here is a PDF copy of it:
https://www.scribd.com/doc/259503500/Bu ... Sports-CarIt starts with a Ford Sierra donor as I recall, but there are plans out there using Miata bits. The thing is 2 inches wider and 2 inches longer and 1 inch taller than the Champion "book" frame (or "+221" in our slang).