Man, that stinks. I've never put an older car on a frame straightener but I imagine it could lead to some issues. Perhaps this link will help you out, it has some original frame dimensions. Perhaps you can use this to help pinpoint the issue.
http://www.corvette-restoration.com/res ... 0specs.pdf
If I had to guess I'd say that your front suspension tower (where the coil spring goes up into the frame) is probably bent upward and toward the center of the car. This could cause the front of the car to be out of balance and result in the car pulling to one side under hard braking.
If it were me here's how I'd try to figure this out (because I'm too cheap to pay for a body shop.
Jack the car up and take all four wheels off.
Place blocking beneath the the frame rails just behind the forward door pillar and below where the trailing arms come into the frame. You'll be supporting the frame at four locations - two front and two rear.
Level across both frame rails and shim your blocking points until the the two frame rails are perfectly level front to back and side to side. A 6' level would make this pretty easy. At this point, provided your frame is mostly straight from the door pillar back, you should have the car sitting more or less perfectly level.
Pull a string running left to right below the front of the car. You'll want the string to be directly below the center of each of the two front spindles. Make sure the string is attached to something hefty and pull it very taught. Use a string level to get the string perfectly level. At this point you'll have a frame that should be level as well as a reference line (the string) that should also be perfectly level.
Using the string as a reference line measure the distance from the center of each of the front hubs down to the string. Both measurements should be the same. If they differ by more than an 1/8" or so I'd be suspicious that something was out of place.
Using this same approach I'd run a string beneath and straight down the center of the car. You can use the frame rails to establish the approximate center line of the car. Use this line to measure the distance to some fixed point on each wheel (say the outside face of the brake rotor). Again, they should match. If one value is different than the other it may indicate that there's a problem.
There are other ways you can take approximate measurements, but this is what came to my mind right off. Hopefully it makes sense and proves to be of some use.
Good luck and keep us posted on whether you figure anything out.