Well my local scrap yard has a naval gun---in two parts---for sale as scrap.  IIRC one of the parts is 40 thousand pounds.... make a dandy anvil no matter how you orient it but the bore is a tad large for punching over...University Surplus from the explosives research department out here...

I think we need a photo of that! Sounds cool!!!!

I have a photo on my home system but I'm teaching blacksmithing tonight for anyone braving the 108 degF temps so it will probably be tomorrow night before I can upload it.

Hydrate PLEASE, on any given day there will be too many anvils around you that if you pass out there may not be any soft landing spots!!!!

here is one of the two pieces with a drink can in the bore for size