You can easliy strengthen a subframe mustang. If you install subframe conectors, monte carlo bar, export brace, and since you are racing, you wil need a full roll cage anyway, it will strengthen it quite a bit. I have seen car's with 1000+ horsepower engines just with the mods I have mentioned.
I hace a '66 Coupe for sale. 4-link, a-arm front. Will accept any engine & is running right now with a 466/C-4. Car has been in the 8's & is NHRA certified to go to 7.50. It's easier to buy one, cheaper too. As for those leaf springs holding 1000 HP??? Not unless you want to spend some real $$$ I'm working on a sleeper '66, 408W with a Vortec "S" trim. About 800 HP & it's going in a stock front chassis, 4-link rear with a 12 point cage. BTW my '66 has a title & tag, head, tail, & brake lights, glass & steel doors, glass front, hood & trunk & has gone a 1.21 60'. http://www.tulsahotrods.com/
click on our cars, 66 drag car. Got a ton of pics, more if ya want 'em!
You can check out this project http://www.carter-engineering.com/ for a tube frame '67. You didn't say if you need such a chassis for drag or road racing. For drag look to Competition Engineering. Road racing do some research on the current Trans-Am cars for ideas/inspiration. You can see my caged '67 at the link below. Good luck.