Gary
First off,that is a fantastic build!! very clean,love the textured 3 piece and the casting numbers,that is one heck of a job!!
The problem is that pretty much all of the pictures Ive seen of Crabs,you can't see the bow gun!The very nature of the flail drive means that whole area is obscured unless it's a 'top down' picture,and those are non existent! I don't believe that they used a blanking plate (like in the Firefly conversions) as there were no extra shell stowage bins meaning the .30 cal wouldve remained in place,but the debris guard made the .30 practically redundant so.........
I personally think your best bet would be to keep the .30 in place,but seeing as you've all ready got the waterproof mantlet cover on,bung a cover on the MG as well,kinda like a 'Crab thats just hit the beach' look but having said that I'm sure that someone is now going to post that perfect pic that will confirm or deny the presence or absence of the .30 cal bow gun (never say never with Shermans!)
And beautiful work on the .50 cal on the turret,but I hate to say it but if you're doing a British Crab,again my (limited) references tend to show Crabs with the .50 removed (but see my last point about never say never!)
Good Work Fella,looking forward to seeing the finished product!