OD for big block fords?

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I have a 77 Mercury Cougar. It has a 351m, which as far as powerplants go, is a sad puppy. It's an engine that gets small block power with big block fuel economy, the worst of both worlds.


I don't want to pull the engine since it works fine, but the 351's status as a big block means that my choices are limited when it comes to alternative transmissions. Problem is, I don't know what my choices are. I've heard something about a 5 speed for trucks that might work, and on the $$$ side I've heard about people getting adapter plates and pulling a AOD from a crown vic, but past that, nothing. I want the most driveable transmission for the buck, and I need ideas about the application and the cost.

Any help, guys?