Guard is by far the easiest.
EW Guild:
Achilles MBT as Leman Russ. (I ended up swapping the barrels to allow multiple variants for my Minervans.
Agamemmnon FAO as Salamanders.
Apollo SPA as Basilisk (If you cut down the barrels to the telescoping point they make good Medusas as well).
Cerberus as Manticores.
Cyclops as Griffons (or Bombards, if you prefer).
Elektra as Shadowsword.
Enforcer UPV as Baneblade.
Exterminator UPV as Stormlord or Stormhammer.
Hector and Paris IFVs as Chimaeras.
Prometheus as Hellhound.
Icharus SPAAG as Hydra.
Tridents as Tank Destroyers (a barrel swap would get you a Thunderer)
Legionaires as Stormtroopers
Snipers as, um, Snipers
EW Khazari:
Valkyrie Patrol (Rocket Pods) as Vultures
Valkyrie Transport as Valkyrie
Valkyrie Transport with Valk Patrol Cannon as Vendetta.
Raiders as Tauros/Sentinels
EW Royal Empire (upcoming):
Behemoths as Titans
Conscripts as Guardsmen
Many of these would be able to be used for Space Marines, too. Zulu-warrior has a nice Marine army log
here using EW stuff.
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SG
Ghost's Paint Blog, where everything goes that isn't something else.