I would say LT1 for a few reasons...

11.6 1/4 mile time shows (in my desktop dyno) to take about 515 flywheel HP in a 3500 lbs car.

At 515 hp you'll have a hard time cooling a 400 on the street especially if its bored and stroked...Thats JMO...

I would say 400 for a few reasons...

515 HP is easier/cheaper to build from a 400 because parts are cheaper for GEN I then they are for the LT/LS's...

With that much power you could really romp it around and you might have enough money left after you build it to put tires on you car because with that much power, you'll be putting tires on about every 3 months