So after some discussion between myself and or GM, and looking at the 40k wikia, we realized that all the dreadnoughts start out with the same Mars Pattern Chassis, and are modified from there.
As for the Venerable, start with a basic Dreadnought, give it the Living Relic Trait from RoB (pg 41), increase its armor base to front 45, side 40, rear 25, a force field with a rating of 35, no overload, similar to the ability that is granted to terminator power armor. Since increasing traits is allowed, we didnt really think it nessicary to increase the WS or BS.
We also came up with a version of the Hurricane Bolter that we think is closer to the idea behind one.
-/3/9, 1d10+9 X, Pen 4, range 75m, Clip 300, Reload 3 full, Tearing, Storm.
Since the Hurricane Bolter is 6 Bolters strapped togeather and fired as a single weapon, the range is more equivelant to an arm mounted bolter and errated weapon damage. The rate is more in keeping with the fact you are talking about standard bolters, and is set to represent one half of the rack of 6 bolters, with the storm trait taking care of the other half. You net the same total rate at semi-auto as is listed in RoB, but you get a better shot total at full atuo by one third.