It's a pretty good custom. I think changing the shape of the base figure a bit to better match the Ark's shape would've made it look better, but it looks pretty good.
I am 99% certain that's an old Zenith Carrierzord from PR:Lost Galaxy. Makes a pretty badass arc. Kinda wish Hasbro/Takara would have some fun and give us an Ark Transformer.
Imagine Decepticons attacking Ark when suddenly Ark slowly transforms while PR-esque music is played.
As the post above demonstrates, Zenith had a robot mode so that's not entirely out of the realm of possibility.