I'm planning to buy a NB2 VVT BP6D engine to refresh with forged rods, ARP head and main studs, ACL bearings, Boundary oil pump . I'll be using a 6258 EFR and plan to keep power levels low enough not to stress my 6speed with a 3.9 diff.

Since I don't have huge power asperations (staying in the 300whp/275wlb max range) or plan to go over stock RPM, I am planning on reusing the stock pistons with new OE rings. I know that for the older generation engines the compression ratio was lower for the Auto transmission engines. I can't seem to find this same data for the NB2 engines. Should I attempt to find a NB2 auto trans engine as my donor or does this not matter for the BP6D?

In case it matters, the car in question is an Exocet race in the 1,650lb range that is used on the street and HPDE. I'm not a clutch dropping guy but sometimes I do get a little to aggressive trying get the current 5 speed box into 3rd so that's probably the most likely way I'll damage a trans.

Its a flying jungle gym that is fast to about 45mph and then hits the aero wall and needs a bigger hammer to punch trough

 

As long as the specs are the same between auto and manual versions (I believe they are) I will ALWAYS choose an auto core over a manual core.
The auto core has not had any of the thrust loads that a manual applies and the auto core has been run at a average lower rpm than the manual core.
Autos in a sports car are for people who don't really want to drive it like a sports car...
For the most part autos have been driven in a "Whimpy" manner.

NB2 pistons are 10.5/1
You're already spending big bucks.
I'd reduce compression unless I was running "heavy" fuel.
Methanol, E85, 110+ racing fuel, nitromethane...

 

What a lot of guys do is source NB1 9.5:1 pistons or better yet 94-95 8.8:1 pistons. They are all the same bore size and a lower CR will yield better results on a BP because the engine will want the timing more than anything and with the higher CR youll be det limited on pump gas.

 

So unlike the earlier NA engines, the NB2 cars with automatics still came with 10.5/1 compression?

 

I think it was only on the 1.6 that the auto was different. They used a 9.0:1 cr and more mild cams on the auto. Try to find some good used NA pistons. They are 8.8:1 up until 95 then it was 9.0:1 after that until the nb1 which was 9.5:1

 

If I stick with OE pistons and the bores in the block are still within spec, I should be able to go with stock rings without having to file them, correct?

 

I mean you'll still need to check the gaps, you'll want to be looser than OEM with boost.