I am looking to get an Eagle forged crank for my 5.3 bored to 3.903.Can i use the 4" crank or will there be clearance issues.I can get a sweet deal on a 3.9" crank but how much lower will that make my displacement?

 

You won't have any clearence issues with the block or cam and you shouldn't have any issues with the piston to counter weight, but no guarantees there. Any of it can be remedied by a good machine shop before you balance it. The 3.900 stroke will drop you 10 ci. Here is a good ci and comp calculator.

http://www.angelfire.com/fl/procrastination/motor.html