1998 Pontiac Grand Prix Engine swap

Call a local junk yard and see if the 98-05 can be swapped.
If they are not listed, then you will most likely have to swap the harness.

When you consider an engine swap, it is a good idea to call a junk yard to see if they are listed as interchangeable.

I will be honest with you, this will probably be a large undertaking. You are going to have to "marry" the two harnesses' together.

You will probably be better off removing the doner harness (everything) from the 05 and start from scratch to the 98.

What you may try is calling a company like Painless wiring and ask the techs if they make (or can make) a harness that will integrate the two together.
You may have to provide them with a schematic of both harnesses'.

There are custom shops that make complete harnesses' but they tend to make them for more popular swaps.

I would not want to guess at the cost.

I will foward this to a moderator who has more electrical experience than I.

One approach is to remove all sensors, manifolds, everything from the 98 and transfer to the 05. The harness should then plug in, but The 05 is a different series than the 98, so there may be some (or many differences between the two). So strip the 05 down to the longblock and put it back together with the 98 manifolds, sensors, alt. AC compressor, fuel rail, EGR, exhaust etc.

Not trying to be vauge on this issue, but generally, when you swap engines, they are generally interchangeable for 1-2 or 2-3 years, in other words, you could probably swap a 99-01 with much fewer problems, but when you skip 7 years you are pretty much in experimental mode.
Also you generally want to swap a transverse engine into a transverse application, not a longitudinal engine into a transverse.

While it may be possible to swap this 05, unless you can find someone who has done it, you will pretty much have to take the trial and error approach.