Not sure which Alan/Allan/Allen you mean. My apologies if it was meant for someone else......Maybe Alan can give me a hand with that.
I realize you are building a fictional vessel but as you are staying with the 17th and early 18th century, the carriages were bed type which were nothing like those for Armstrongs, and Blomefields. I only have a few pictures on my laptop while we are traveling so apologize for not being able to provide more information.
Assuming you want something from English ships circa 1650-1710
The carriage design below is from the HMS London wreck 1665 Carriages in 1725 were still bed type but a little different.

The Commonwealth barrel pattern would be most appropriate.

As to dimensions I have nothing specific but once you have the length of the barrel, be it the designated or the overall length, the carriage dimensions can be figured out pretty closely. I would be delighted to help as much as I can IF you wish.
Allan