my friend has this 400 and its in good shape and he has these lt1 heads just laying around, just wondering how hard it would be to make the two work together?

You have to fill in the water ports that are already in the head, drill and tap so they can use the gen 1 water pump, do a search for LT1 head conversion it's been done but took alot of machine work.

Here is what I would recommend:

1) Sell the LT1 heads.

2) Buy an upgraded Vortec head kit from sdparts.com.

You'll be dollars and miles ahead going this route.

Vortec heads are better than the LT1 heads. The aluminum LT1 heads are OK, but not as good as the iron LT1 heads - and the Vortec heads were a design based on the iron LT1 heads, then added more improvements and they are compatible with Gen 1 engines already. (I'm assuming the heads are stock, unported castings).

I'd go with Vortecs over LT1 heads on a Gen 1 any day. Same or better performance for much less trouble and money.