The great work continues guys - fantastic work all. Thought I would pop in a quick update before running to the office - I have the M4A3 decking basically done on my Jumbo build, and will start adding the applique panels next. Here is the hull work compared to the unaltered kit parts:

The details for the side plates were cut out and sanded down as one would with vacu-formed parts to allow them to be reattached. Weld beads were added from stretched sprue and, in the turret guard area, epoxy putty. Up front the welds have been relocated forward, a replacement antenna pot cover plate made and relocated forward and the hatch hinge areas filled to better match the real thing. Looking at the pictures I can see that I have left off the welds crossing the side plates at the back of the turret guard and the engine door props, but you get the idea.
The Hobby Boss engine deck must be separated into three panels on either side of the engine grilles to allow it to be narrowed, the foremost and rearmost solid panels being narrowed from their outside edges and the grilles being narrowed by chopping hinge frame areas off and narrowing the grilles themselves, then reassembling them. Bolt heads are from stretched sprue and grab handles from .012" brass wire.
While I was at it I scraped off the spurious (for an M4A3 - the kit depicts an M4 or M4A1 with a Continental radial engine) detail on the hull bottom and replaced the engine hatch with the proper part for the Ford GAA powered M4A3 made from .010" styrene with stretched sprue bolt heads. You can also see the nearly finished transmission cover, thickened to match photos.

Here is the top side of the cover:

Lots of fun here, hope to continue to make progress, but have to start working on finishing the models I am taking to the AMPS Nats.
Best to all -
Konrad