Hello,
Host
2003 Monte Carlo SS 3.8

Donor
2004 Monte Carlo 3.8

I'm in the process of swapping my original transmission with a donor. but the model is different. My Original is tag as 3LDB and the donor is 4RNB. would there be any modification that needs to be done as the donor(W0491) is non W pin. as for the Original it is (W2T1W). I read other forums but does not give great details. thanks

At a minimum you'll need to reprogram to accommodate the swap:

3LDB is designed for a Buick 3800 and has a 3.29:1 final drive ratio
4RNB is designed for a Chevy 3100 and has a 3.05:1 final drive ratio

I believe there are other differences including the torque converter between those two applications.

Host
2003 Monte Carlo SS 3.8

Donor
2004 Monte Carlo 3.8
Are you sure the donor trans came from a '04 3.8 Series II engine? If the trans was actually fitted to the 3100 cc Chevy engine, the bolt pattern would not fit, as well as other major components & wiring. Have you seen the donor still located in the original '04 3.8L MC? Very confusing here, Some Ting Wong going with this transplant?

@CathedralCub I was able install the transmission without any issues, alignment fit correctly. I was able to shift through each gear, but when trying to place it back in park it makes a strange noise. Would I need a wiring harness that I can replace so it has 19 Pin. several manuals I read that some 2004 transmission have extra switch in the park neutral position switch, but they do not include the W-pin. To my understanding the original transmission is an External mode switch, and the donor is a internal.

@hotZ28 correct it was off a 2004 MC 3800 (I remove it at the junk yard), the tag says 4RND, I was able to install the transmission to the engine without any issues. Unless the tag may be incorrect. I agree but when i read several articles i seen that some 3100 transmission from 04/05 Century, and 2002 Century/Grand Prix will fit. i'm not sure how much of it is true.

They were changing a lot of things through those years. I've seen best luck swapping same year and drivetrain for same year and drivetrain. Even then sometimes there are differences.

You can only trust half of what you read on the internet! I only give info here, based my knowledge gained & memory of 50+ of experience.