Mike, I will post more Firebird articles on the Titus cars as time allows. I suspect you may have them but others more likely do not and would get some useful insight by looking them over. Interesting that the rear frame is so poor and inaccurate on the Hwy 61 piece that you're having to go to the trouble to section in something from the Yat Ming version. It makes me want to go track one down to see how badly they botched the job. I see the Hwy 61 chassis uses the Turbo 400 automatic transmission crossmember. Are you able to source the correct crossmember from one of your other kits? I like that you are building enough extra parts and pieces to allow yourself some options right up to the end. I think that's a good idea.
Somewhere, I've got a two-car scale model package of the '69 Mark Donohue Camaro and the '70 Parnelli Jones Mustang that may or may not be Hot Wheels. I forget, as I have not looked them over in quite awhile. Looking at your 1/64 '71 Javelin on the shelf, my thought was that my two cars looked like they could be part of that line of cars as I do believe two other cars were available --- a Javelin and a Challenger. However, I never did purchase those two.