I have the 3100 in my car and it hasn't been the worst engine, but its got its faults. The 3400 with a 3400 transmission will hold up FINE if you're just looking for a high mileage everyday driver.

My father just bought a 2005 Impala with the 3.4 and he told me he's never owned a smoother, more efficient engine. So the swap would work, just get a transmission to go with it.

I've learned on these forums, if you don't have a 3.8, good luck...