The 2-bolt block is the way to go if using a stock 400 block. I'd go with the 350 5.7" rod if using stock rods. If you're planning on using a stock 400 crank, I'd external balance it because it'll cost a bunch to internal balance due to the addition of mallory metal. I ran a 406 with a stock block, stock cast crank and stock 350 rods w/arp bolts with 13:1 compression and a little solid flat tappet cam, World iron sportsman heads for years in a 3000 lb 63 Nova and never had a problem with the rotating asm. It ran 11.20's at 118 mph in Colorado in 8000 ft air.....