HR,
I am having the same problem putting a 292 in my 80 model C-10. My service manual shows the passenger side frame mount is definitly different between the mount for the 250 and 292. I went to my favorite wrecking yard the other day to look for one and was surprised to find he only had about a dozen cars/trucks left. I guess scrap is paying better than used parts. I'll be getting out the torch and making my own.
Anyone have a Hollander manual that would show all the years that share the same mount?

Willie