There is something that draws me to long slide 1911s. I'm not sure exactly what it is but I like the extra sight radius and heft of them. My favorite recreational pistols sport the long slide and barrel and I shot one in bullseye competition for a while. Building one is really no different than building a standard government model except for three specialty parts that are unique to these guns.
Obviously, you'll need a long slide. Caspian Arms of Wolcott Vermont has been supplying their slides to custom pistolsmiths and manufacturers for decades and their customers include some of the biggest names in the industry. Available in carbon or stainless steel, Caspian slides can be ordered with a variety of sight cuts, cocking serrations and engraving.
You can also choose from round (Colt style), serrated, knurled, snake skin or plain flat top profiles. As an old-school traditionalist, I prefer the Colt style round top and vertical rear cocking serrations.
Keep in mind that these are gunsmith fit parts made to maximum rail dimensions, so you will have to fit a custom slide to your frame. Usually, working the slide on to the frame with some lapping compound will result in a nice fit.
Six-inch barrels are available from Kart, Nowlin, Clark, Bar-Sto, Nighthawk, Les Baer and probably other makers. Some, such as the Clark, will have to have the large diameter at the muzzle end reduced behind the bushing for proper function. It is possible to build longslide pistols in any caliber you find the 5-inch guns chambered for. Like low recoil? Try a 6-inch 9mm pistol. Do you want a pistol for hunting? Try a 10mm.
Springs and Plugs
Since the 6-inch slide is longer than a Government Model, you're probably thinking about longer recoil springs. Forget it. Stick with standard Government Model springs by using a special spring plug with an internal shoulder that shortens the spring tunnel length by one inch. These long spring plugs were sold by Rock River Arms for years but I don't see them listed on their current website. Caspian Arms lists them. Order one with your slide.
Aside from the three proprietary parts listed above, you can use standard items available anywhere for your build. Now go build yourself a "big iron" 1911.