how hard would it be to swap out a lt1 or ls1 with a 427 bigblock. what would need to be changed and what is needed. roughly what would it cost

 

Horrible idea.

You'd murder most 427 bigblocks out there with a cam-change and longtube headers in an LS1 (around 480 horsepower) and still get 25mpg on the interstate.

 

agreed.

It MIGHT be a reasonable idea; IF you had a v6 car with a toasted motor AND the 427 BB sitting around. But even then, the extra stuff youd have to buy to make it run would be just as costly as JY 6.0l stuff with a cam and some exhaust upgrades.