Difference between 2.9 4 and 3.5 6
 

99 has a 3.0 six cylinder rated at 190hp. You have a (swapped) 3.5 six cylinder rated at 255hp. Depending on what year your 3.5 came out of, you could have an oil burner on your hands.

Frank, what part of the planet do you live in?
I know that the 4 cyl version exists.

Most of us are in North America. To the best of my knowledge, only the 6 cyl version was sold here. It's the engine which we are familiar with.

I have owned 3 Nissan pick up trucks with the 4 cyl engine. They have been very good engines. My son drove his 1994 pickup past 270k miles before he sold it. The engine still had a lot of life left in it. The late 1980s and early 1990s model had bad timing chain guides. The engine is bullet proof once that is done.

You would have 130 to 140 hp. Probably considerably less power than you have now, but with better gas mileage. Decide which choice is appropriate for your needs.

Confusion
 

4cyl
 

Replacement struts are not very expensive. You might want to replace them.

the 6 cyl in a V design, wide engine. The 4 cyl would be an in-line engine, a narrow one.

You still didn't tell us your location.

We had a member in Turkey with the 4 cly engine.

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Oh, it's advertised on Craigs list and the owner thinks it's a 4 cyl? Either he is flipping the car, or knows next to nothing about it.

Caution then regarding maintaince issues.

It's a 1999. Those have issues with the alarm system . That often means a tow to the Nissan dealer.

Confusion
 

:facepalm:

Location
 

Yeah, he must be related to a mechanic at Brake Check that was trying to convince me that the Nissan Maxima comes with V6 and V8 options! He must've spend too much time in Colorado!