If you have a 3.0L now, no question the easiest swap is the 5VZ-FE 3.4L and I would definitely not consider the Vortec in your case.
Stock the 5VZ-FE produces about 190HP and 220 ft-lb of torque, it's a solid little motor, especially for a mini truck. This would be a significant improvement in the power of your truck. Your 3VZ-FE produced an optimistic 150HP stock but they've never to me really felt that powerful. You can add the TRD supercharger to get about 60ish more HP. Keeps it stock and things fit, sensors do what they're supposed and stuff like the tach still works AFAIK.
I guess it depends on what you want to do with the truck, but doing a small block is IMHO opening a whole can of worms on breaking stuff, not to mention the question of mismatching and adapters. The R151F probably can take a mild 350, though, and Advanced Adapter does make a bellhousing to do it. You'd have to fabricate motor mounts, probably do custom radiator and hoses. Not sure how the gauges connect, but I think most people just do a custom dash with a small block conversion.
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