There is a professional sand blast outfit on the border of Rowan County NC, They do lots of Nascar stuff, chassis bodies everything... They did my Coupe and no distortion at all. If you read Flops posts you get an idea of how well the panels turn out. Last time I was there they had a complete 67 fastback ford mustang ready to go in and get blasted It was stripped and just a shell. I specifically asked about warping etc and he told me the only bad thing was the hood and roof ribs would leave "shadowing" as sandblasting does actually remove a little metal. I was very pleased with the stuff he did for me. Of course I brought him the pieces all striped, ie, loose quarter panels, cowl, subframe all in knock down form... I guess it depends on who you hire and if they know what they are doing?
http://www.bigrblasting.com/