Lead on a 408. Should I do it?

Stick with the 340 , at least you'll know what goes in to it . Don't worry you'll be making close to 400 hp . Good set of pistons , air gap headers and a decent cam will get you there . I punch it at 10-20 mph and light up the tires , put down 50 feet of rubber hitting 2nd gear , all with a little 340 .