This was a conversion idea that occurred to me yesterday. Though I won't have the funds to pursue it for a while I thought I'd put it up here on the off chance that someone might be able to make use of it.
The Leviathan Dreadnought is a Forge World model I adore. I love me my Dreadnoughts, in fact part of the reason I'm collecting a Blood Angels/Flesh Tearers army right now is that they have so many Dreadnoughts. The problem with the Leviathan (aside from the Forge World resin which is evil) is that the body is £47 and then you have to pay £13 per arm on top of that.
So, how to make one for less and in plastic, aka a material that is actually workable and comes with some sort of quality control.
Firstly, the basis: a Primaris Redemptor Dreadnought. One of the great design features of the Redemptor is that that massive front plate with the aquila on it is designed to swing up to reveal the Dreadnought's sarcophagus. If you built it leaving the sarcophagus exposed and perhaps replace it with one from the Venerable Dreadnought kit to give it the helmet present on the Leviathan.
As to weapons the plasma incinerator cannon would make a passable substitute for the Grav-Flux Bombard whilst the power fist could be augmented with any number of claws (perhaps blood claws from the Furioso).
Hope that helps.