Need to make a decision fast 5.7/6.1/340

This spring i bought a 70 duster with a tired mild 360. Now after a summer of my foot on the pedal she is real tired. Im debating on throwing a 5.7 hemi in for performance and economy. I want a street/strip car but i also dont want to pass everything but a gas station.

The guy i bought the car off wants to sell me a 340 with 9.5 pistons, steel crank, with headers and he has a set of edlebrock heads that need to be machined. He wants $4000 for the setup but the way i look at it is thats $4k for a used motor. He claims around 3k miles on the build. I also want more horse than what he claims in that motor. Im shooting for 450hp

Im womdering if someone who has done the 5.7 swap can tell me any pointers they have with problems they ran into and a round about price to do the swap. Im not looking to change the K member or anything makor like that. I PLAN ON putting my. 727 behind the 5.7