351D:
The magnum heads would be a good upgrade to your 351D, but you would also need the magnum intake and exhaust manifolds to match up to them. They should be a bolt up swap.
I am unsure your source of information for inability to get parts. So far, I haven't had a problem, but it does sometimes take some effort. Some sources you might try are:
www.kanter.com Mills Supply (Shreveport, LA)
Fliteline Engine Supply (Fort Smith, AR 1-800-
726-1427 ASK FOR BILL)
Napa Auto Parts
Also, check the group I referred you to. They are very helpful.
Regards,
Steve
