Those fins are actually pretty stout unless heavily corroded. Try a stiff nylon bristle brush in a small area I think you will find that you can proceed with vigor. You might try contacting Huron Valley Sales. They are big RayPak dealers and might even have the ability to email you with an illustrated parts break down. If I recall I could actually remove the fire brick in my RayPaks and then replace them in the same order after cleaning the tubes. Maybe the brick only came out because they were so old. One thing to look for is sagging tubes in a RayPak. A sure sign of over heating. If they are sagging you best replace them after going to all of this trouble to this point. The manifolds will start leaking soon if not already. In fact a leaky manifold could actually cause corrosion from the steam generated when the antifreeze drips onto the burner. You might also want to remove and clean all of the burner orifices since you're in there anyway.
Chim Chiminey, Chim Chiminey, Chim Chim Cheree, a chimney sweep's happy, as happy can be!