I am not aware of any original fowler that did not have a tapered barrel.
The "Flintlock Fowler" book divides them into several groups such as New England, Club Butt and etc. All are distinct and different and many are big bored and very long barreled. Interestingly enough it does not cover trade guns of the same period.
Perhaps I am over engineering this question a bit. If you want a smoothbore that is comfortable to shoot but not locked into and given period of time or location; that is different than if you wish to portray a specific persona, such as a colonial hunter in the New England marshes.
I prefer a gun style that I like and is comfortable to shoot. If a gun is fun to shoot you will shoot it more often. Unfortunately the best way to tell is to try different types, which is not always easy and you run the risk of picking the wrong gun for the time period you wish to portray.
Likewise, you may find a 1750’s fowler that fits the time period you like but just does not feel right.
I think the advice given so far helps explains options available. Personally I lean towards the 20 ga. plain Jane trade guns.
At any rate a gun is a sizable investment and it is good to asked and be well informed before entering the market.
Good luck and keep us posted.
