Zieg, The swap as a whole is rather easy. I question what transmission in is the Fleetwood...200R4 would be my guess. I'm wondering what the Fleetwood mounts look like? Think big crossmember & an engine/trans assy. basically in the same general placement as the truck, I would think they should work fine. Probably need to drill a few holes? If not, there is several ways to do them. I've used the Olds parts on the ones I've done, however I know of guys who have used the Chevy ones & made a plate to attach them to the Olds block. Here's a couple pic's...I just made a flat plate across the 3 holes in the block & 2 studs to put the mounts in a nice place on the truck crossmember. The old Diesel set-ups used a plate similar on the Right side as I remember.
The rest of the swap is straight forward...alter a wire wire length/placements, head pipes, should move the battery (but not required), ect. An active weekend should do it.
Now should you should be putting in a 185,000 mile 307...that's the REAL question! Will be glad to help with any questions you would have if you decide to do it. Take Care, Lorne