There's no point in worrying about how to measure the length of the gun barrels in this case. Regardless of how you do it, there's no guarantee that the manufacturer of the aftermarket part did it the same way. There is, unfortunately, no substitute for getting the aftermarket part in your hands and comparing it to the drawing of the real thing.
There's no universally applicable rule for using the "next larger" or "next smaller" size of gun (or any other component) if you can't match the ones in the kit exactly. "When in doubt, err on the small side" is a good general-purpose standard, but you may find that the "next smaller" size of gun barrel either looks ridiculously small or doesn't fit the kit carriages. Again, there's no real way around the problem other than to get a sample of the aftermarket part in your hand.
That was a nice feature of the old-fashioned, well-stocked local hobby shop. You could take your kit part in and compare it with all the available alternatives. Ah, for the Goode Olde Dayes. We should, on the other hand, remember that in those Dayes the number of aftermarket parts available was much smaller than it is now. I question whether it would be possible in this day and age for any one store to stock more than a small fraction of what's being produced for all the aircraft, armor, modern warship, and sailing ship modelers - to say nothing of the model railroaders.
The carriages in the Revell kit are tapered correctly. The ones on the maindeck are simplified, one-piece moldings; they're difficult to see on the finished product. The one long gun that's fully exposed, the bow chaser, has a nice, multi-piece carriage. The carronade carriages are simplified a bit, but not bad.
Ship modelers have had lots of interesting discussions about the color of rigging line, and there's certainly room for argument - and personal taste - about it. In the British Royal Navy of the eighteenth and early nineteenth centuries it was a matter of law that all rope purchased for the use of warships be soaked in Stockholm tar. I've never seen Stockholm tar, but it's reliably reported to have been a rich, medium brown in color. That's appropriate for the running rigging of a British warship of that period.
It seems to have been standard practice to "blacken" the standing rigging, after it was set up, with one of several concoctions that contained tar, lampblack, sulphur, and various other disgusting, smelly things. If the rope treated that way wasn't pure black, it must have been pretty close.
I know of no reliable documentation about the color of rigging line in the U.S. Navy of 1812. But it seems reasonable to assume that the Americans did such things about the same way the British did. Lots of modelers (including me) use brown thread for the running rigging and black for the standing rigging. Two golden rules about ship model rigging: 1. When in doubt as to color, err on the dark side. 2. When in doubt as to size, err on the small side.
An iron-stropped block would be a rarity in a ship of this period. In reality the typical block-and-strop assembly of the late eighteenth or early nineteenth century would be a rather complex piece of marlinspike seamanship. The strop would be a piece of rope spliced into a circle (called a becket), soaked in tar (probably), and then seized around the block in such a way that an eye was formed on one or both sides of it. (From the middle of the nineteenth century onward, there might well be an iron thimble in the middle of the eye.) The rigging line would then be passed through the eye and have an eyesplice worked into it to hold it there. There were all sorts of variations on that theme, but that's the basic idea.
I think it's safe to say that, at 1/96 scale, few if any modelers try to represent block strops separately. Buy the rope-stropped variety (i.e., the kind that just has a groove around it), and secure the rigging line itself around the block. Actually, the blocks in the Revell kits, if carefully painted, would represent blocks with rope beckets reasonably well. But even the smaller size in the Constitution kit is mighty big for 1/96 scale.
CapnMac82's suggestion about getting different sizes of rigging line is an excellent one; using a wide variety of diameters greatly improves the look of the model without increasing the amount of difficulty or time involved. The number of different sizes in the real ship would have been in the dozens, but three to five will make a huge difference in the model. If you use a "marker" to color the line, read the label on the marker carefully. Some of those things are, to all intents and purposes, permanent; others start fading in a matter of weeks.
Hope that helps a little. Good luck.