I also strongly agree with Mark.
I was going to reccomend the SHP block last week but before posting rememered that SHP only comes in 350 main journals.
If you already have a 400 block and ratating assembly you have the large journal mains, so you would have to start all over again with a new crank to fit the SHP and rebalancing your rotating assembly.
For all that, I would just step up to the Little M Dart block with 400 mains, and you will have an awesome block to build on for many years to come (that is if you don't put windows in it...?)
Yes, it is a few more dollars now, but will save you a lot in the long run and you will have a brand new block of new design rather than a 30 year old block on its last leg...