If you are looking for very detailed drawings, Antique Ordnance Publishers (AOP) has large (11" x 17") format drawing sets for many U.S. Civil War artillery pieces, limbers, and related stuff. I used these to make or repair bits and pieces for a friends cannons used in re-enacting.
http://gunneyg.info/html/AOPCatalog.htm
Probably quite expensive to get mailed.
Field Artillery Weapons of the Civil War by Hazlett, Olmstead, and Parks has a chapter on Parrott Rifles, with some dimensions and photos.
It's worth understanding that the US Civil War involved a long protracted conflict with lots of supply issues. Most models are reasonably accurate generic representations of a specific style weapon, as the barrel dimensions are fairly consistent and the carriage designs were as well. Over time however, you could find all sorts of combinations as barrels failed, carriages were damaged, and weapons were lost when positions were over run. If you are seeking historical "accuracy" the best you can do is either build from original government specifications and prints so your model in as new and delivered condition (kind of ignoring supply issues and contract mods), or you model a specific example at a specific location matching things up to photographs or AOP drawing made from original data and a specific example.
I've heard purists bemoaning the fact that the National Parks in the US have barrels on the wrong carriages, wheels are the wrong diameter for that carriage, this detail or that detail is "wrong", all the rest of that. In some cases it's that bits and pieces were thrown together well enough to fill out a line of guns when the park was built. In other cases it's likely bits and pieces were throw together by soldiers to keep as much firepower available as possible, without caring if the rope on the trail was three or four strand.
The guys trying to move a gun cared more that there were wheels on the carriage than if they were 52 or 54 inch diameter
Have fun,
Stan
