I am planning on buying a new DART block for my 74 Nova rebuild and I am putting in a 3.75 stroker crank in it and I know that is the crank for the 400 *** block but should I stick with the 350 mains since it comes bored 3.990 or am I thinking too much and I should go with the 400 mains since I am using a 400 crankshaft?  

Have you bought anything yet?  It doesn't make sense to buy a block with 350 mains and then a crank made for 400.  If you wanna use the stroker kit just buy a block made for 400 mains...