The Olds 403, 350 gas, and 350 diesel all use the same engine mounts available new at Autozone for $9.99-18.00. But the 1973 GM pickups with Chevy gas engine frame might not have the holes drilled into the frame to use the Oldsmoble engine and mounts, as it was made well before the 1978 year the Olds diesel was used in any GM product, and part numbers for the Chevy gas engine mounts used in '73 and the Olds late 70's series engines are different.
Measure up before comitting to the swap, as it could be more work than finding or building a good Chevy engine. The frame holes could be there and match up, but you better be sure.

-Fordlords-