What would have been the visible external differences between the flamethrower sherman and the 75mm? Would the flamethrower have a bigger muzzle?

I think that they used scavenged 75mm barrels to disguise the flamethrower tanks. If that's true, the only teltales would be a sooty muzzle and an access hatch cut into the top(?) of the barrel close to the mantlet. I'm sure that one of the Sherman experts (I'm not one) will chime in with more detailed (or accurate) info.

Gday HB
The flamethrower Shermans most prevalent (although not many in Europe at least) used the bow .30cal position for the flamethrower, designatd E4-5. Used mostly in M4A3E2s,the bow 'barrel' looked slightly shorter, but that was the only external difference... Apparently not worth their while though, when attacking pillboxes on the Siegfried line...
There were but a handful made as the Sherman 'crocodile', which had a large unit attached to the antenna pot, top right of the glacis, and of course towed the trailer as per the Churchill crocodile...
They eventually required the assistance of the Churchill from British squadrons for bunker busting as they were far more effective.
Hope this helps some
Cheers
Brad

The answer to your question is there is no difference at all.(I am assuming you are talking about Pacific Sherman Flame Tanks?) The Flame Projector was fitted in a old 75mm Gun Tube and it looked the same as a regular gun tank.
Cheers
Jeff

Well, yes and no. There is no difference between the barrels, however marines used the M4A2 for their normal 75mm tanks and M4A3s for their flame tanks.

I was reading up on the Flamethrower shermans, and that the ones in the pacific the main guns were removed in place of the flame gun.

That lead to the Korean war version (some made it to WWII, but close to the end of the war) which mounted a 105mm howitzer and the flame cannon mounted co-axial to the 105.