In addition to Clifford Performance and Langdon's Stovebolt Engine Co. already mentioned here are some other sites devoted to inline 6 performance and parts.
Inliners International devoted to all Inline powered vehicles
Autowerks - 250/292 products
Info about lump heads for the inline sixes
Bobby, don't discount the capabilities of the Chevy sixes, they're great little engines. Cams and other parts are available to really make 'em go! For an example, the attachment is a screen shot from the Howards Cams catalog with the listings for 194, 230, and 250 sixes. (In these listings the "Note 1" at the right means "Stock (O.E.) valve springs can not be used".