I am assuming you got a great deal ... if so and you want to get the best performance and minimum modifications, get it rebuilt (or, do it yourself) ... crate engine is likely a bit too expensive since it would cost as much as the car with a good engine ...

Good luck ... my 91 3.5 is still running great ... only thing I have done out of the ordinary is remove intake tract and clean it up, disable the EGR (btw, I still continue to believe the close placement of the EGR and the valve cover venting to the intake are partially at issue as they cause the buildup of gunk that eventually breaks off into the intake) .... the low end torque is quite a bit better than in the earlier 3.0 engines.

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George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg