The main basis of the conversion is just the front half of a Spartan model wedged between Fellblade sides. The magna-melta from a Caestus Assault Ram I had sitting around fit rather nicely into the Spartan Heavy Bolter turret (though one can bash one from a pair of Spartan exhaust pipes pretty easily also, which is what I'll do to replace the Caestus' gear eventually). The Fellblade side doors were filled in and a 1" drill bit then used to create the recesses for the front flamer sponsons converted from a pair of GW Sentinel flamers.
For inspiration on the upper superstructure I looked to (and used parts from) the Sicaran tanks. The front coupola from a Sicaran was mounted to the right side with the remainder boxed in with plasticard sheet. Various components from the Sicaran Venator roof were cut up and arranged on the top of the superstructure to look 'busy' and to frame around a standard size razorback turret ring. It was then sheathed with additional layers of plasticard on the sides in patterns inspired by typical 30k SM vehicle features.
I wanted to keep the fellblade rear exhaust system but needed clearance for the rear access door so elevated the exhaust stacks to the upper structure, but extended the piping down along the rear access sides. The door doesn't quite clear the pipes swinging straight down, so I explain it via a complex hydraulic system hidden inside which drops the door down first to clear before swinging out.
Various plasticard bits were added to detail out the deck area, with Tichy Train Group rivets liberally added throughout. The fuel tanks are a design I created which I add to all my Fellblade chassis, based on the type sold by Blood and Skulls Industries for the plastic Baneblade. http://stores.ebay.com/Blood-and-Skulls-Industry Blood and Skulls has my master for the 30k Fellblade fuel tank - if enough people ask them to produce them maybe he'll add them to the store.
In all I'm very pleased w/ how it turned out, and just have a couple touch-up areas before it gets painted up for my son's Death Guard army. The only down-side I've found is the doors are a bit low for a Contemptor Dreadnaught to assault out of - but I've built a solution for that also: http://heresy30k.invisionzone.com/index.php?/topic/7852-mastodon-released-today/?p=146664