Hi,
I have just finished assembling my frame as I am doing a frame off resto. My frame looked very scary when I bought my vette.
I didn't know if I was doing the right thing when I bought it. When I lifted the body there was a lot of surface rust and holes on two parts of the frame that later on I took good care of.
I put a good epoxy primer and a scrath free top coat on the frame that is against salt and sweet water.
To be honest, I think the best way to protect your frame is to do the frame off resto, ass you can coat the complete frame.
But maybe you dont have the time for that. So see if you can get the car on a hydraulic car lift(the ones the service shops have) and take a good look on the rust maybe its all surface rust as most vettes of that year.
Look for holes created by rust. Get all the rust off and also clean all traces of oil and grease.
Then try to coat with Por-15, don't forget to coat the inside of the frame. That may require a special tool.
Good luck,
Ryan