Hello, this is my first day here on the forum.
I have a lot of experience with American made engines etc. This will be my first Nissan.
I am going to put an RB25DET in my 1970 240Z.

Do I want a NEO or not? Any particular years to stay away from?

I am going to put a Tremec TR3550 behind it. My question is whether the balance of the engine is dependent on the flywheel as some American made engines are? Or is buying an aftermarket flywheel and bolting in on okay?
Should I be certain to buy a manual transmission donor engine?

The AWD to RWD conversion can be completed with just buying an aftermarket oil pan and pickup. At least that is what my research tells me. I see a lot of people welding up the AWD pan but I need a rear sump anyway.

A non turbo is 10.5 to 1 and the turbocharged is 9.0 to 1, I think. Are there other differences like block strength or oil return bosses. I have experience with turbos and injection tuning.
If I bought a non turbo engine and replaced the pistons designed for a turbo engine will that drop the CR? Are the heads different? I am sure the cam has a different grind.

I am not asking for you all to do my research but if you know right of the top of your head I would appreciate the help.

Thanks.

Quick answer: Start off with a DE-T. They're plentiful, the engine was designed for boost, and converting a [rare] RB25 is going to be a colossal PITA. The heads and block are indeed different.

I have had an RB in a '72 Z for 12 years, so you're on the right path. NEO or not isn't crucial, I prefer the earlier engines (if you can find one). Typically, they'll need re-sealing and a good inspection, but they're pretty bulletproof. Any straight-6 is an inherently well-balanced engine to begin with, so either FW should be fine (no real reason not to use the stock one).

You shouldn't need to worry about AWD conversion - That was the RB26DETT.

Make sure you get as much of the original stuff for the engine as you can - ECU, harness, MAF, anything you can get. Sure, you can source stuff, but a bone-stock proper-running RB in a 240Z is a recipe for frequent tire replacement and loads of fun.