Yeah, I don't see this working very well... Oblits are a little larger than a Terminator, while Helbrutes are a little larger than a Dreadnought - comparatively:
Honestly, my favourite conversions involved combining
CSM and Beastmen kits - even if it works out about the same instead of cheaper, at least they're less crap.
I'm debating making Centaurish Obliterators if I ever do (probably won't as I'm all in for Defilers / Forgefiends for my Heavy Support loadout)