A few questions.
 

If replacing an already LS1 swapped FD with an LS3, is it plug and play?
Does an LS3 require a different ECU than an LS1 (not referring to tuning, but the ECU itself)? (I assume it just needs a tune?)

For new swaps, do folks prefer to use standalone, or a stock Chevy ECU (because OBD2)? Is either considered an all right way to go?

I found references to the serpentine belt pattern, corvette (wide+short) vs Camaro (narrow+tall), and some people saying you should use the Camaro pattern.
Is the Camaro pattern required, or just easier?

Various swaps use different swap kits. I see many require just a tiny push into the firewall to fit the motor. Then I saw one swap where the motor is recessed really far back, only half of it visible inside the engine bay. Is there a reason to heavily recess the engine? (Trans shifter fitment? weight balance?)

Thanks,
-scheherazade

Most use GM ECU, I have an LS3 with LS2 GTO ECU.

If you haven't been to norotors.com, get thee there and search around a bit, there's a lot more V8-swapped RX-7 info there.

LS3 has almost all different sensors and all plugs are different. ECU is different. You'd be better off buying a harness kit from PSI to run an LS3.

Stock GM ECU is easiest and cheapest option.

All serpentine setups will fit basically, but Fbody is most popular because it is the cheapest.

All the engine mounts sit about the same. There is only room for it in one spot plus or minus a half inch. You dont have much options for engine placement like you're thinking.

Easier and better to use the GM ecu. Guys can make 2000 HP daily drivable cars on the factory ecu and tune with HPT or EFILive. As for the swap wiring and sensor placement are very different between the two. Just depends if you want the DBW or cable throttle body. I prefer wire over cable for the tunability, cruise control and traction options. Also packaging is easier.

Your biggest problem is that the LS1 uses a 24x reluctor wheel on the crank, and the LS3 used a 58x wheel. Also LS3 is drive by wire. This means a convertor or different ECU is needed. Additionally, some of the sensors are in different places, but that is easy to fix.