For the apparent cost, if you are able bodied, it almost seems better to buy even a cheap welder, chop saw and mandrel bends and learn to do it yourself. Way back when I looked for shops for that stuff, shops that could mandrel bend, were quite a bit more expensive so doing it yourself can produce a nice system at much lower cost.
If you can manage to piece together a system for someone else to weld, then you have done the majority of the work. Get a welder, if possible and take care of the rest. It's a bit of a cliche at this point but it's also true, watch some Youtube videos to learn how to weld. Anything you want to learn is there.
2" pipe is about the largest that can work with rams horns and is larger than necessary for most Y Block builds. So make it easier on yourself with 2" or smaller pipes. From what I can remember, a socket would not work to tighten the nuts with my rams horns and 2" pipes. Only an open end wrench would work to tighten the nuts. So 1 3/4" to 1 7/8" might be best, particularly if otherwise stock. Probably go with whatever fits the flanges, as they are provided. Smaller pipe is also easier to route so you do yourself a favor in that respect if larger pipe is not necessary.
Lawrenceville, GA