Thanks for the comments. While cruising ebay for bits I stumbled across a starwars prepaint figure, the Raxus prime trooper, that might provide a decent helmet with a little work if it scales up close enough.
For those interested:
The legs are from current GW space marine terminators. I clipped off the thigh plates and filled in the thigh armor with some greenstuff.
The torso is an assault marine torso with the stomache cut out and wings/skull bits on the torso cut/filed off.
Then I take the termi legs and cut/file down the bulb that the terminators torso is suposed to go onto so that it fits the normal sized marine torso.
The stomache area was resculpted with greenstuff to help extend the torso a tiny bit, then I stuck the "belt buckle" bit form the original torso onto the terminator legs where it looked the best.
At this point it looks a little bit wasp waisted, so I pack on the grenades and or pouches etc for some added bulk.
arms are for the most part plastic space marine arms.
The back valve thing is a bit form the pegasus hobbies chemical plant kit
The shoulder pads are a normal space marine pad with the edge trim filed smooth and 3 cuts to shape them into a more t-51b-ish shape. Of course I didnt want to do all that work multiple times, so I made a very simple 2 part press mold out of 5 minute plumbers putty and cast up some more pads with it using greenstuff and/or more of the 5 minute plumbers putty.