The issue with trimming down 45 Colt brass that far is that the farther you get from the case mouth, the thicker the brass gets. Even though it is a straight cartridge on the outside, the interior tapers slightly, with the case wall getting thicker the farther you go. Trimming the brass down that far it might be a real problem getting a good crimp.
As I said, if your cylinder will accept 45 Schofield rims, that is one way to go, the Schofield has less case capacity than the 45 Colt, and can more easily be loaded down without issues of too little powder in the case. Starline makes 45 Schofield cases, I load them all the time for my 45 Colt revolvers. But the rim is wider in diameter than 45 Colt, so you may have an issue chambering the rounds in your R&D cylinder. You may not, I had to open up the counterbores in my cylinders to accept Schofield rims, but I believe R&D has changed things slightly now so that their cylinders will accept Schofield rims. It depends on when yours was made. I can measure my counterbores and get back to you to give you an idea of what mine were opened up to to accept Schofield rims.
Or you could call up Adirondack Jack and buy some of his 45 Cowboy Special brass. It is specifically designed to do exactly what you want. Provide a cartridge with the same case head and rim as the 45 Colt, but in the same length as the 45 ACP.