Anybody know why i cant run a SBC 400 with LT1 heads with my TPI set up?
P.S. Does anybody know what cc Stock LT1 heads are?

Because the LT1 heads are for a reverse flow coolant system. If you look at the coolant passages on the LT1 heads compared to the 400 block the are different. Also look at the front of a LT1 block compared to your 400, also different.

I think there is a way to have your block machined to use LT1 heads but it's expensive. At that point i would just buy a set of AFR's.

LT1 heads have 54cc chambers. WAY too small for the big bore of a 400ci. You'd never get the CR down to a reasonable level. (even with the stock pistons a 350ci LT1 had 10.4:1 - and they used quite a thick head gasket).

LT1 heads require heavy modifications to the heads to make them work on a Gen 1 small block. You don't have to modify the block at all. But with all the costs and hassle of having them converted, you could easily buy some killer aftermarket small block heads that will bend over a set of stock LT1 castings. LT1 heads are nothing really special in stock form. Its when they are ported by someone that knows what they are doing that they turn into world-beater heads (near 300cfm/210cfm isn't uncommon from AI or LE ported heads)

You can get the benefits of the port & chamber design of the LT1 with Vortecs.

GM Fastburn heads are known to make some pretty good power.