The '96 & up SPFI 3400 was virtually the same motor as the '94 & up 3100 (not to be confused with the MPFI 3.1 LHO motor), just slightly bored out (with the same stroke), and 1.76" intake valves. All 3100s also got the same 1.76" intake valves from '00 on up, so only the bore was different between them at that point, making the 5-10hp/tq difference between a '00 & up 3100 and 3400. This is why a lot of early 3100 owners swap over to either 3400 or '00 & up 3100 top ends. It will gain you another 10 - 15hp/tq over a pre-'00 3100. No, it's not huge, but for what you can pick up those motors for these days... In fact, if you can find one cheap enough, you're better off snagging a whole 3400 motor, trans and wiring harness. That will net you around 25 crank hp more than your current motor if you get one from a later year car. For bolt-ons, FFP used to make a smaller underdrive pulley, which was good for around 7hp, and billet dog bone mounts with poly bushings. They no longer produce them, but used or NOS ones pop up now and then in the classifieds on w-body.com.
Other than that, there's not much you can do to the 3100 for a significant increase in performance short of boosting it.
