1/2" Chuck on 3/8" Cordless Drill?
You're looking for Jacobs part number 31037, it's a keyless 1/2" chuck for standard 3/8" drills designed specifically for the purpose of converting them. Your 3/8 drill will be fine with a 1/2" chuck, I've got a few than have been converted, 2 air powered, 1 corded/electric and 2 cordless.  Just treat the drill as you normally would, drilling holes larger than 3/8" with a 3/8" drill has never been a problem.  

I converted my first cordless drill for the same reason, needed to use an Irwin Unibit with the large shank.