Re: header gasket leak, 401 crank in a 304
The headers usually leak around the dog-leg in the port. Stare and compare the remains of the old gasket to the header. Sometimes welding around the outside of the tube and reshaping the inside to better match the head will do the trick. Factory manifolds will not cost you much in performance and will seal much better.
The 401 crank will fit into the block, but you will need 401 rods, 401 rod bearings and custom pistons. The main journal size is the same but the rod journals are larger on the 401. The rods and crank are much stronger, as they are forged.
If you don't find bad mains or rods in the 304, pull the cam and check the timing components, cam and cam bearings for wear.
Hope this helps, good luck!