[LMSRACER]

Whenever I'm building a 400 Block Based, Performance Engine, I look for Blocks that haven't been drilled or tapped for four bolt main. This way I can install Splayed Caps on the Center Three.

Also, whether I'm building a 400 or 455, I prefer a Block without Cylinder Bore Chamfers. This way I can put the Chamfers where I want them. That's the problem with the '67 400 Block, the Bore Chamfers are too large and they are in the Wrong Place for '68 and newer cylinder heads. (Some '67 Cylinder Heads Too.)

As for the Main Webbing Issues, that is more of a 400 Block Issue than a 455 Block Issue. In almost every case, the 455's Main Webbing is much thicker than the 400. Look for a 1970 to 1974 455 Block.

The 400 shines in the "Less Likely to Crack Down the Center" department because the 400 is 1/8" (0.125") thicker from the edge of the Cam Bearing Tunnel to the Edge of the Main Bearing Tunnel. This is of course due to the 3.00" Mains in the 400 compared to the 3.25" Mains in the 455. Again don't confuse this thickness for an Increase in "Main Webbing" Thickness. They're NOT the same thing.

The Cast #481988 400 Block that hasn't been Drilled for Four Bolt Main is a Good Candidate for a Performance Build. Although the Main Webbing Does Vary from Year to Year.
The Cast #'s #9799140 and #485428 455 Blocks are good Candidates for a Performance Build. I actually have a #485428 that was never Drilled for Four Bolt Main and now has Splayed Steel Main Caps on it. It was Engine Code "YC" to Start with and it's Date Code is B221.

To Wrap it up, with all of that being said, In most cases the Cylinder Wall Thickness is what Varies the Most..... Once you locate, what you think is the "Right Block", it very well may have Thin Cylinder Walls. Sonic Testing ALL used Blocks is a must.....

Have a Good One,
Larry S.