everyone seems to think tubeliners are the goto AI.
Not true. There are some of us who either delayed getting into FSX specifically to ensure we could port over our Calclassics AI planes. So while it's not as "mainstream" - there are options, though some cases involve modding (or in my case, just accepting) the FS9 port over issues. I don't have a single modern tube in my FSX.
Places to start:
Calclassics - they are mostly focused on FS9, but with programs like AIFP these can be easily transformed. They have recently updated their AI section, and are constantly tweaking 50s propliners - some flyable, some AI.
Historic Jetliner Group. Again, focused primarily on FS9, but many of their flyable planes work superb in FSX. More to the subject at hand, they have JUST started releasing AI versions of their DC-8s. I suspect they'll be extending out to mirror in AI what they've done in flyables - so I'd expect B707s, B727s, C990s, etc. Not propliner, but nice for the 60s to the 90s.
Military - Many choices, and extensively discussed in many threads - chiefly here:
http://www.sim-outhouse.com/sohforums/showthread.php?77258-FSX-native-military-AI-aircraft
Additionally, MAIW - though also focused mostly on FS9, can be easily adapted into FSX. Michael Pearson (and others) alone has built the entire A-4, F-8, F-4, F-104, F-102, B-52 model range - to name a few. AI F-105s and F-100s are currently in development. The fact that they were intended for FS9 is irrelevant - they work fantastic in FSX.
Needless to say, between John Young, Rob Richardson MAIW, Calclassic and HJG, you could build buckets worth of AI that's "retro" - a silly term that clearly means different things to different ages - but you get my point.
Cheers,
DL