Since the exhaust you already have is set for long tubes ( full length ) i'd just run them. 1 5/8 primaries, 3 inch collectors are pretty standard. My other truck has 1.5 inch primaries and 2.5 inch collector headers on it though. You will most likely not notice any difference , especially you have nothing to compare it to. For your application, it's nothing to worry about. Again, for your application, placement of the h-pipe won't matter as long as you have one -you'll not know any difference. Sounds silly but it's real. You will still reap benefits of having headers and connecting both sides of engine exhaust. It'll sound REALLY cool AND make better power than the next guy.
The vast extent of my experience comes from cars that sit much lower to the ground than trucks, which is in part why I specifically made mention of behind the transmission. Mostly because that's where the space is in cars. One could weld collector fittings in the exhaust anywhere they felt like so you could unbolt the entire system (or any part of it.) For example, my hotrod has long collectors and fittings at the end of the headers, no tailpipes, and I can unbolt the h-pipe and mufflers from the car without having to break my header collector seal to remove/service the trans or I can run it with no exhaust ... lol...An h-pipe has an inherent amount of HEAT to consider and I want it away from my transmission.
Full length headers, long collectors, extra collector fittings. This runs as long as the transmission. My h-pipe sits directly underneath the front of my driveshaft, not the yoke. A very small piece of pipe before the mufflers, mufflers and resonator tips that terminate before the differential.
I think kind of a general rule of thumb is the closer to the engine, the more benefit one will realize, but if you're anything like me...i'm constantly messing with it and it MUST be practical (simple to service). Tuning my collector length was more important to me for my race car and I wanted to be able to unbolt it. My lowered hotrod is driven to the track, it ain't no trailer queen and ground clearance IS an issue.
If it were ME, and I had an exhaust for full length headers, I would get some. They are really pretty cheap. I'd put it all on the truck and take it to an exhaust shop. The exhaust guy will probably have a very good idea and some experience with your truck. Get it jacked up and look at it, the right spot may prove obvious. My exhaust guy charged me 40 bucks to do it with aluminized pipe. We had a piece of pipe, held it up, marked the spots on both pipes and he simply cut a hole in them with his welder and made it. Remember though, mine has extra collector fittings in it and being serviceable was mandatory. As soon as you connect both exhaust pipes, the entire system becomes one very large piece (and it's heavy), especially with full tailpipes. I just want to help man, good luck with whatever you decide to do, I don't wish to overstep a stranger's bounds.