Will a 2005 3.1 buick fit in a 1999 buick century with 3.1

No. You need a numbers matching engine to make it plug and play, or rebuild the stock engine. Remember the rest of the car is still old and the transmission is most likely going to let go once you have a good engine in place. That why it’s call opening a can of worms.

you can transfer parts from the original existing block to the transplant block. be sure to be very careful an keep everything intact to keep parts to transfer to the next engine. i got a rebuild put in my car at the dealer shop... it seemed that they just transferred to old parts that are good to the other engine.

in retrospect in my case , i should've rebuilt the heads and timing chain. my block was flawless.