Oddly the 3.9 never seemed to be as reliable as its v8 siblings. Around here a used replacement would be a little tough to find. You could always grab a reman from a parts store.
I don't know, My '87 is at 170K, and if it weren't that Trusty Rusty got to the point I wouldn't sticker it again I'd drive it anywhere w/o a 2nd thought. Even with the 2bbl carb.