I assume there are many parts that are same the same. I would believe that alll engine related (Pulley, T-Stat, Rockers ect are the same), but what else ?? FWI/CAI? PCM? Belts? Any Exhuast?
Just looking for more part buying options/ places.

PCM and belts should be fine, though IIRC the PCM would need to be flashed. Same physical unit though. Exhaust on the Bonnevilles is a bit different than the Regal or GP. We've found that the manifolds and downpipes aren't interchangeable.

An FWI or CAI will probably also need to be modified to sit in the Bonneville engine bay properly.

I thought one of the belts was different because of the accessory bracket differences?

the accessory belt is diffrent do to slight change in alternator positioning. I think the belt is shorter for the gtp.

So if I found a GTP Press on 3.4 pulley it would fit. How hard is it to put on a press on pulley? What is a good price on a used 3.4 pulley?

My bad. Everything else I said should be valid, though.

Given I live in Michigan and it is starting to cool off. I am going to do a 3.4 press on and a 160-180 T-stat with a drop in stock style K&N.
Will this be a safe set-up over winter with more supporting mods to come in the spring?
What belt will I need with the new pulley?
What is the thought on intercoolers? and what kind of kits and results are there?
Does anyone make a Lowering Spring Kit for the bonneville Will I need Camber caster plates?
Thanks

Unless there were changes in the routing you'll need a 715 belt. Trust me

You'll still need to scan to make sure ya don't have any KR. The next mod will definately need to be the exhaust to open up and get better flow