I say go ahead and put the 400 in there. Just use heads with centerbolt valve covers and keep a stock looking injection system on the motor. There will be no way for a smog shop to tell the difference between the 350 and the 400, unless the guy knows the older block had a driver side dip stick and a 2 piece rear main. If you're really worried about it, grind the casting numbers off of everything and copy the numbers from the old block onto the new one using JB weld. Paint over them and nobody will know the difference.