My Fleetwood has the 307 olds engine, not sure which transmission, and I'm wondering what other engines could I put in without too much trouble. The 500 is idealistic, but I have no idea how easily it would go in, how well it would mate with my tranny (if at all), and how much stuff could be recycled from my 307 (water pump, power steering pump, etc).
I would like as many details as possible. Any suggestions as far as which engines have the best compromise of power/reliability/ease of installation?

FWD or RWD

Check out the RWD section. There are a some posts about this swap that might help you and already have the information you're looking for.

It's a RWD.

any Buick olds pontiac caddy motor will bolt up to the trans. motor mounts will be a different story. not all that much more work than any other motor swap.

Go the easy route like I did. Swap in the senior version of your Olds 307, the Olds 455. You can reuse your mounts, accessories, and nearly everything while easily tripling your horsepower. The engine shares all the same lower block dimensions, but has a raised deck (approx. 1" higher). The top of the engine is about 1" wider per side and can be kept to the same height if you want to use a Toronado intake manifold.

In my Seville I used W and Z manifolds from an Olds 442, the original motor mounts and an Edelbrock Performer 455 manifold and everything fit. Your car should be even easier.

Go For It!! affordable and quite fast.

Check out my swap page and my main page link:
http://hometown.aol.com/wadewmbryant/455.html