I just bought a 3800 Series II for a Jeep CJ swap. The guy I bought it from is a mechanic and he said he pulled it from a RWD Olds. But how can I tell for sure? all I have is the block and heads.

The engine doesn't care if it'* in a FWD or RWD.

There was never a 3800 Series 2 factory installed in a RWD Oldsmobile.

But I though the motor mounts were different, the oil filter placement was different, and the accessory mounts were different?

Either way, there was never a 3800 Series 2 factory installed in a RWD Oldsmobile.

If your mechanic pulled it from an Oldsmobile then it is FWD.

Personally I know of no difference between FWD and RWD short and long blocks. I looked around on the Internet and found no (credible) others that know of a difference.

Then I found this thread which also seems to think that there'* no difference: https://www.gmforum.com/performance-...vs-rwd-239733/

You might want to go back to the guy you bought it from and ask what model and year of Oldsmobile it came out of.

There are factory RWD 3800 Series 2 engines. Yes, they do have different intake systems and oil filter mounts.

Wow, a google search and the first result is me actually saying the same things...

But . . .

. . . are the same, right?

Correct. The block and heads are the same.

During my quick research yesterday, found someone had to re-TIG a different snout for the M90 supercharger to work in a RWD environment.

Interesting, for hood clearance or belt alignment or . . . ?

The inlet of the M90 supercharger faces the firewall and apparently gets pretty close to it when its in a RWD environment.

IMO, I wouldn't mind gutting a hood just to make a RWD 3800 L67 fit. Heck, I'd make an electric clutch for it just like Mad Max.