Why not a 413 ?

Milling and steel strapping the main caps along with main studs along and about 1/2 inch of hard blok along with some very liberal piston and ring gap clearances would adequately strengthen the bottom end. The factory forged cranks are super strong, too. There have been some early to mid seventies motor home 413s found with HP2 (six pack) internals including the external balanced steel crank.