360 build in a 66 Cuda

Look at the engine number on the front of the block. If it came out of a bus it will likely say 360-3. The -3 engines were the factory HD truck motors. What does that mean to you? Not much. You may have the extra steam/water holes on the deck. Otherwise, perhaps exhaust valve rotators and a steel cam sprocket. You're not gonna be reusing most of that stuff so it matters not. You didn't mention compression, are you replacing the pistons? Either way that cam should give you a boatload of grunt in a 360, perfect for a tall gear like the 3.23's.

Get a real converter, don't use the worn out /6 converter. You are using a later 904 trans right? Didn't the spline count change in 68?

The standard gears for a mid 60's /6 A-Body were 3.23's and if it is the original rear, its a 7 1/4 (the 8 1/4 came out in 74 for A-Bodies, and will have the big bolt pattern). The 7 1/4 will "howl in protest" behind a 360, especially a mild cammed stump puller like you seem to be building. It'll hold up on the street for a while, but will be decidedly fragile if you drive aggressively.

RPM will not drop with a V-8 if you change nothing else. In fact if you recycle the converter, they will be slightly higher due to increased input torque. If you plan do do any freeway work (80+mph cruising) 3.23's are about perfect, and I wouldn't consider going deeper than 3.55's. A low gear set and a good converter should help you move off the line.