I've got a 2000 gmc 5.3 with 260k. Been drinking oil so found a 6.0 but its the cast iron heads and longer crank. Question is ...can I swap cranks is all that the same internal?

 

Yes you can swap cranks. Get rid off those cast iron heads and get a set of 317 heads, they will be better over the cast iron. Internals are slightly different, but I'd say they are close enough for what your trying to do... Someone correct me if I'm wrong

 

Someone over on ls1tech supposedly used a 6.0/80e flywheel and drilled holes for the 60e.

Seems like a lot of hassle to swap cranks.