The early (606?) built IM's would be the same as FF/CMC (since CMC bought IMs tooling in 1978) . Looks like entire frame is there - with lots of rust holes. Rusty Tubs used to sell a frame (with fiberglass floor) that looked like CMC one (when they were in ATL). They still have web site but not sure they exist after their move to Bolder City NV. $395.
Porsche 356 kit car floor pans JUST LIKE ORIGINALS! (rustytubs.com)