I swapped a 3.8 Galant engine that I converted to DOHC into my SR. I expect I get the generous side of 250Hp but most of the power is up high. Over 3500 this thing pulls hard to 6500 where it shifts, but down below 3k it lacks power. I just towed a trailer to Conn and back and I find my aging Mighty Max pulls much better. The big problem I have is the wide gear spacing in the automatic in the SR. That transmission just does not let the engine work where the much less powerful engine in the lighter Max can be used efficiently..

If I were to suggest a change for you, get a used 3.8 engine rather that rebulding yours, change to 4.63 gears to reduce the big step to overdrive or keep the 4.22 and swap to a manual transmission so you car use the power the engine makes.
I have a 6 speed manual waiting to go in my SR, maybe in June when I bring an employee on I can get the swap done. This is not a simple swap but the 5 speed from an earlier Montero or from a Sport will go in easily.