I have to agree with Chryco. That's a lot of money to be spending on a 360. Plus the work of swapping things out. Plus having to try to sell a motor home that's now 318 powered. 360's really aren't that hard to come by in the $200-$500 range.
I know you're looking at the mileage, but consider the mileage that's put on a motorhome, especially anything heavy. You're looking at mileage that comes in spurts and if it's hilly out, then you're looking wide open throttle most of the time to pull the hills. This after usually sitting for months at a time. Think about it: it's a '77 with 20,000 miles, that's an average of under 600 miles a year. That's basically one trip a year, equivalent to me going to Boston. How was it stored for the rest of the time? How did it fire after all the sitting? My guess is that it didn't fire right off, it took a lot of pats of the throttle, leading to a lot of cylinder wash, leading to a low mileage engine in need of rings and bearings. What were oil changes like? Once every three months during all the time of sitting or once every 3000 miles... every 5 years in other words.
As has already been pointed out, these are low compression smog engines, and sitting in a motorhome there's too many unanswered questions regarding the use and storage. No where near worth your time or money.
This isn't a deal. Pass it by.
