Unfortunately there may not be a cost effective option. I would take it to a local radiator shop to have them take a look at it. I had mine done last year and it cost me $800 ('84 Chevy 454) to have it recored (used the original tanks). In my case there were no aftermarket alternatives since the tank is custom with the neck on the front of the top tank rather than the top. It stung a little in the wallet, but it's one of the better investments I made in the thing. It runs nice and cool under all conditions now. 

Welllll I just had mine done in my 1975 and after extended discussions it was decided to have mine re-cored. Seems the copper fins cool better than the aluminum ones. That's my two cents. It wasn't all that expensive. I took it out my self and put it in my self.

I got to tell yeah mate if you got any mechanic worth his salt and he's a good one. Spoil him. Good Mechanics are hard to come by today.