I'm on holiday, so I'll keep this short.
Forget about anything larger than 338, unless you want to design and fab a full custom gas system. 358 doesn't leave enough meat in the barrel to be reliably threaded for the M14 pattern gas system.
I have a machinist who runs a CNC machine willing to contour bbls. Runs of 50+ would be the only way to keep it under $250/bbl.
A plain Jane 1.25" 26" blank will run just shy of $400, plus tax and shipping.....
Contouring would be $350-400 unless lots of 50+ are ordered.
Instal, would be $275.
Added up, best case scenario: budget on $1000 installed. Custom one-off, budget $1300.
John