You could not build a .577 NE on a P14 Enfield, but you could build a .585 Nyati. However, it certainly will not be a budget project because the labor will cost more than just buying a magnum Mauser action new for $3K.

A Ruger No. 1 can be rebarreled to .577 NE and you would only have $12oo to $1500 in it. That will leave plenty of funds still available for chiropractor visits to get your neck re-aligned after shooting the rifle.

A relatively affordable big bore option is a .600 Overkill on a CZ550 built by American Hunting Rifles www.hunting-rifles.com for $3500.