Dennis,
Your earlier comment about the Euro model not being the Ajax got me to looking around. I found that a consensus among UK, German and French modelers is that it the model is from plans drawn by FA Chapman found in the Swedish National Maritime Museum.
http://www.sjohistoriska.se/en/Search/ChapmanXXXII While it is not clear that this particular plan was ever built, ten of Chapman's frigates were constructed for the Swedish Navy in the 1780’s as the Bellona Class.
http://www.ageofsails.de/ships/fregatten/bellona.htmThe Euro kit could probably be bashed into one of them without too much trouble.
An request to the Swedish museum might get you these plans and even some paintings to set the colors.
Just a suggestion, finishing your ship as a Swedish frigate would make a more colorful, as well as more interesting and accurate model.
Or, as one of the Bellona frigates, the Venus, was captured by Russia and served many reasonably well documented years in the Imperial Navy, you might want to do it up in Russian colors.
http://www.bruzelius.info/Nautica/Ships/War/SE/Venus(1783).htmlWasn't US Northwest a Russian colony sometime in the late 1700s?
You have done a very nice job on the model so far and will be a great model however you flag her. Sorry I didn't get to you before you started the coppering...would have been unnecessary on a Swedish frigate of that time. But the Russian Venus spent a lot of time in the Mediterranean where copper would have been essential so maybe you can still recover the coppering time.
Seems that the Kit has also been attributed to a French frigate la Renommee. Chapman spent some time in French dockyards so his plans could have been copied/influenced by a French frigate in the stocks at the time of this visit.
See
http://modelshipworld.com/phpBB2/viewtopic.php?p=193136 or
http://www.modelships.de/French-40-gun-frigate/French-40-gun-frigate.htma tangled web indeed, but part of what makes ship modeling so fascinating
Will Gasser, Zurich