A bunch of stuff to address, so this will be kind of long.
As promised, here is the GCR rule from the SCCA 2003 rule book.
Paragraph 17.1.4.C (pg ITCS-6) says in part:
To maintain the stock basis of Improved Touring, updating and/or backdating of components is only permitted within cars of the same make, model, body type (eg., sedan, station wagon, convertible, etc.) and engine size as listed on a single Improved Touring Specification Line. Any updated/backdated components shall be substituted as a complete assembly 0engine long block, transmission/transaxle, , differential/axle housing.) No interchange of parts between assemblies is permitted.
...(TGSI NOTE: that takes care of the FI system. It goes on.) ...
No interchange of parts between assemblies is permitted, and all parts of an assembly shall be as originally produced for that assembly (such parts may, however, be painted or plated). Additionally, it is not permitted to "create" a model or type of car by updating or backdating assemblies.
... (TGSI NOTE: it goes on to say a bunch more about the manufacturer "superseeding" parts are OK...
So, we are not creating a new model. The Manta 1.9L was/is the model. We are not creating a new type of car... we are not creating a Yugo nor an electric/steam/flux-capacitor powered car.
Would any year Manta work just as well? Yea... considering all the stuff it takes to turn it into a front running ITB it really doesn't make a lot of difference. It's just easier to get everything all at once and know it's working to start with... then replace it all. The FI rules require a stock harness too so that would be a lot easier than creating a new "stock" harness. In reality, very little of the stock stuff will get used without being replaced or having significant changes... I don't even care about the computer except for the housing. The rules allow anything inside the stock housing.
The '75 does also have the "big brakes"... but I would go with only new rotors and calipers. I could just as easily "update" an earlier model, so earlier brakes don't really matter. I did just think of one thing. The early Mantas didn't have the big 5 MPH bumpers and they're ugly. So, I guess any Manta would be fine.
Rally Bob... Come over to the dark side? I guess that I've always been on that side since I've always liked the Mantas a lot. It's just that they could never legally be made to run anywhere near the front. As far as handling goes, I think a properly done GT and Manta will be about equal. So the real difference will come down to the FI. With a real engine management system (electromotive) stuffed into the stock housing, the FI will probably make enough more HP to bring me completely over to "the dark side".