You might want to increase the gun's diameter to 155mm ^__^. [Edit] done ^__^
Not sure if it was more important than say a Priest (and hey, I would prefer the Sexton).
You might want to wait until the M4A1 comes out, I remember Rubicon mentioning they were doing the M3 style bogies on the M4A1 kit (which might allow the Grizzly to be built from it).