Put new fuel tank in and now when I try to drive it dies but only under power.

poss line problem kinked while installing tank, double ck

you might have to drop the tank and check for a kinked hose or a loose connection

Brand new tank, or salvage yard tank?

Why was the tank replaced and did you also replace the fuel pump?

Sounds like a lack of fuel pressure. i agree with the above mentioned items. curious why tank was replaced and not fuel filter- could be loose connection, kinked hose or damaged strainer intake on pump.