Unless I was really on a budget I would use the 4l60e, since it has a low first, a lockup, robbs less power & is compatable with the 5.3 ecm.
As far as I know they are all 27spline.
Are you sure you want to go with a 5.3? I just got my exahust done and modified my TBI ecm with the EBL from dynamicefi.com. So far it is pretty sweet and the EBL with TBI should support well above 500hp. As a plus, my stroker is compatable with off the shelf SBC speed shop parts so its no problem getting any needed parts locally.
I should be updating my thread soon with info and pics. keep myour eye's on craigslist and you could find a build SBC/BBC that you could swap most of your TBI parts into. Just a though
Feel free to post any questions in my Hardcore forum thread.