I just joined and haven't read all the postings but have seen this come up a lot. Having built a very healthy 410" 400, I've found a simple way to get any compression you want in a 400. Because the 351C and 400 share every piston dimension except wrist pin diameter, and the 351C has been very popular in the racing circles for years, there is all kinds of aftermarket pistons available for them. The only thing you have to do to take advantage of this for your 400 is to bush the small end of your rods to the size of the Cleaveland wrist pins.(about .060" smaller than the 400, I believe), and run full floating pins. I did that on my engine and intended to get something around 9.5:1. Unfortunately, the machine shop decked the block a little more than I expected and I ended up with more like 10.5:1 instead. With the open chamber M heads I get some detonation at the top of 2nd and third when I'm really flogging it. It doesn't seem to be too bad if you don't do this too much, but I intend to see how it is with racing gas and a little more timing. I could also double the head gaskets but this makes me nervious. I have this in a 79 Ranchero GT and at 4250 pounds, it still scoots! Rear tire wear is a real problem! The only real concern I've heard about with these engine is the strength of the block. I guess if you don't use squeeze or try to build it too wild, you'll be ok. A main girdle could help, too.