I have a 1999 Ext-Cab 1500 Silverado with both stockelectric seats. the Lumbar and back are manual.
I can get apair of 2004 Chevy Avalanche fully automatic bucketseats/w complete rails.

Are they a direct swap replacements. Please give details of any concerns or special difficulties

You can most likely bolt them in, but that's about it. The avalanche seats have different/more connectors for the power options. Most of the upevel trucks with power seats this new are controlled by the drivers door module with memory controls/etc. If the seat rails don't match yours, forget about it all together. You can't unbolt the rails on those like you could the old onesbecause they're part of the seat bottom and all the power motors are on the rails. Icould be wrong, take a closer look, but the last set of rails I changed on an avalance seat was a pain in the butt all because oneof the power seat motors died.