What makes the most sense to me would be putting in the same type of engine (Late Model Toyota V6) and going forced induction. If they make a TRD super charger for a 3m-Zfe that should be an OK plaform. You can do pulley upgrades and meth injection and stuff.
Maybe a 5-speed swap from a camry. Maybe do a built auto if you want a drag Sienna.
If not the 1mz should fit fine and they have a s/c for that. The 3m makes more power but both are good motors.
look at tony the tiger he has a custom (expensive) big single turbo set up on his V6 Camry with 400 something horsepower. Is that enough?
That would be way easier than tube-framing a mini van