I see that 1970 Chev trucks could be purchased with either a 250 or a 292. Chevy did not typically make two crossmembers for a truck series.......this swap has been done a zillion times so there is an easy answer......if we can just find it.

What about the Chevy truck parts folks that are always sending catalogs out?.....forget their name.

Anyway, there is probably a frame plate that has an extension built into it so that the motor mount bolts to the block in a normal fashion and does not torque the rubber in the mount......I will look to see if I have any old shop manuals that show an exploded drawing.