I find it very hard to imagine that a whaler would spare deck space and cargo capacity to run 18pdr guns (the only options I am aware of are the 9ft 42cwt, and 8ft 38cwt iron guns, and the impractically expensive 5ft9 18cwt brass guns (recently removed from service after their failure at Badajoz), along with a few other types of brass guns - Brass is extraordinarily expensive as a gunmetal compared to cast iron in this period.)

The description of "long" is instead, I believe a description of the later pattern ~8 cal carronade/gunnades, rather than the shorter patterns (of recently cast examples ~6 cal, but with some of the very early 1780s pattern pieces being even shorter than that.

Atalantic/Atlantic/Essex Junior is usually described as armed with 18pdr carronades and 6pdr guns.

As an additional point of data - the insurance guns for RN transport board hires were "At least six carriage guns or six pounders or less bore as the board shall think proper according to the size of the ship." There was gun money of £5 per gun as an inducement to carry additional guns if the ship would bear it. Carronades of at least 12pdr were an acceptable substitution for the 6pdr guns.

The small size of whalers and their crews doesn't suggest long and heavy naval pattern guns are likely, but carronades are ideally suited to the service.