He 111 bomb load permutations

[edNorth]

From what I have seen, so far, tanks were built in on assembly - i.e. versions stayed the same, unless "modified out" (for training use mostly)

[Denniss]

AFAIR four internal bomb cells (one side) could be replaced with a fuel and oil tank.
The H-4 was just a H-3 modified to carry larger external bombs.
In theory both H-5 and H-6 should be capable to carry full internal load or bigger bombs on external mounts.

[edNorth]

H-6 had more powerful engines, the Jumo 211 F/J, and larger Wooden props, so in theory was superior to the H-5 (VDM Alumin props and Jumo 211 H-1 engines); which in case of the (15) 4./KG 40 machines (1941) had been fitted with 2x internal tanks (in both sides of cabin) and all bombs externally.