I strongly agree that Masao's pennet & emblem designs be used. He has done an incredible job of designing Starfleet and enemy ships of earlier periods in a consistent and original manner that complement the designs of th Original Series. I have recalibrated the Spaceflight Chronology timeline to put it in line with more recent developments in Star Trek Chronology, and the Baton Rouge class ends up having been launched about the same time as Masao's Valley Forge class.
http://www.starfleet-museum.org/valleyforge.htm
While that ship looks somewhat different in design, the overall volume of the Baton Rouge is about twice that of the Valley Forge, so it would play the role of a Battleship (which would euphamistically be called a Heavy Explorer by the Federation). So, one would not be suprised by differences in engine design required to move a much larger vessel, one who's design origins probably date back a decade before launch. The mass from the Spaceflight Chronology has to be corrected up to 277,500 MT to come in line with Masao's designs, such as his Siegfried Class Dreadnought.
Star Fleet Battles has the following registries for refitted Republic Class vessels (they may have a more detailed list in their "The Early Years" expansion product, which I do not have yet).
NCC-1371 Republic
NCC-1372 Reshadije
NCC-1373 Ramilles
http://www.starfleetstore.com/MERCHANT2/merchant.mv?Screen=PROD&Store_Code=S&Product_Code=5623
While your design departs from the fan designs that came before for the missing secondary hull details, fan based Treknology, and more professional resources are now pointing towards the class receiving multiple large scale upgrades through the years. I have it in service from 2226-2266, though these dates may change by a year or so in the upcoming revision. The Republic Class appears to have been a somewhat later related design to which the Baton Rouge Class was uprated, so if you are concerned about copyright issues, that class is an option.
http://www.starfleetstore.com/MERCHANT2/merchant.mv?Screen=PROD&Store_Code=S&Product_Code=5724
As to whether the Constellation class predates this class, it can go either way, either the Federation allocated blocks of registry numbers to certain types of ships, the designs of which might not be built for many decades, or the Constellation and sister ships were major refits of vessels that predate the Reuplic Class. SFB has the names of some converted Constellation Class vessels as well (which might also have been expanded upon in "The Early Years" expansion:
NCC-1017 Constellation
NCC-1018 Goeben
NCC-1019 Eximer
The Defiant was on my list as well (1016), but Enterprise has pre-empted that registry, at least for the TOS era ship, though the registry could be used for an earlier ship. Presumably, these were all originally Horizon Class vessels (NCC-1000, as provided in other Treknology sources).
Getting back to the model at hand, it is a really beautiful piece of work and you are a master of the craft. Please keep posting on your progress. Congratulations and good luck.