As mentioned, piece of cake.
4L80E's came behind 6.0L gen III GM V8's in probably millions of 2500HD's, they bolt up just like a TH400 (because TH400's and 4L80's both made it on the GMT400 platform, I'd guess). So basically you need a flexplate from a 4L80E (if you don't have one already) or a simple spacer (which is actually all the made the 6.0L/4L80E flexplate different from a TH400 anyway).