5 or 6 speed auto on a 360??

The parasitic hp loss from a 5 speed auto would probably terrible.
I'm not positive on that, but it's my best guess having a 4 speed auto now.

I like having the overdrive as it facilitates a lower rear gear, but just that one extra gear contributes to roughly 23 percent driveline loss.

A 46rh, or a500 would be a decent choice for you.
This site has conversion kits to control the lockup on the converter and overdrive automatically. You have to scroll way down but you'll find it.

I think the 4 speed auto and a lower set of rear gears in a light car like yours would be fun. Especially a manually shifted one! But like I said earlier, the power loss is a factor you need to take into consideration.