Yep 390 4.05 bore with a 3.98 428 crank. 66-67 Mercs had them. Used std forged FE rods and the 428 cranks are nodular iron not forged steel. It's rumored some pickups came with them as an unknown option. I've never seen one personally. The 410 pistons were used in the pickup 390's tho. With a higher wrist pin location this was a solution to getting a low compression truck engine without new piston tooling. Denny is the resident 410 expert. I'm sure he'll tell ya more!