Thanks for all the replies, it's just about impossible to reach all the surfaces inside that area. I can get 2 fingers in there, that's how tight it is. (easy now)
I've used everything I had, and even made some stuff.
My sander with a 1/2 x 18" belt worked the best of everything I tried.
I made an attachment for a die grinder which was essentially a 1/4" rod welded into a piece of tubing with a slot sawed into it.
I put about a foot of 2" or so sanding belt on it and let that flail around up in there. It must have done something, it made a lot of dust!
I've ordered some of that Eastwood frame coating to shoot up into there, maybe it'll last another 47 years.
By then I won't be caring much about it.
Thanks again for all the ideas.