Figured I'd put in my $0.02...
I've done sand blasting and have both warped panels and had it come out perfectly. When I was back in high school (10 years ago) I blasted the rear fenders of my '47 Chevy coupe with play sand. With using the sand, being inexperienced, and trying to be thorough I warped both fenders, one really bad. Since then I've blasted using coal slag (available in 100lb bags at the local hardware store for $13) on the cab of my '55 3200 2nd series and it worked extremely well. I was told to avoid the roof because that could easily warp, but everything else would hold up just fine. I've also been told that play sand is extremely cancerous and you should never blast with actual sand, only blasting media (coal slag, glass, walnut, etc).
As for the frame, I've just been going through mine with a wire brush and Por-15, and it looks amazing and seems like it will hold up extremely well. I've been working on the same gallon of Por-15 for almost 10 years and it has been well worth the price.
Good luck with the blasting! I'd love to see some pictures
