I agree with CrazyMark and Felix. Having the boolit already in the die and perfectly aligned with the GC shank dia portion at the bottom would be a way I would trust to produce repeatable results. Actually, given a gently angled step from major to minor diameter, I don't see how it could mess up. A few years back, a friend and I made up a two-dia size die to bump the Lee C312-185-1R nose portion to the right dia for his rifle. Running the driving band portion into the minor dia was also no problem at all. A size die and heel post to match. Sounds simple to me.
Scrounger's idea of a cutter to skin the heel band down to GC shank dia would work also, but how would alignment be maintained? This suggests the need for other pieces of tooling to accomplish that part. In the past, I tried to skin some .430s down by lightly chucking them in a drill chuck and holding a file against the heel band as they spun, to get the dia down. It worked, but precise consistency wasn't my forte. A collet would have been a huge improvement over a drill chuck, but t'was not to be.
There is another low-cost low-tech gas checking technique for a Lyman 450 that worked well for me in the past, and it can be done on any flat base boolit that is several thousandths larger than the desired finished diameter. It involves a little machining, but only two pieces and doesn't use a conventional GC:
1. Machine some round stock (half hard 4140 or grade 8 bolt?) so the OD is a close drop-in fit inside the die retaining nut on a Lyman 450. Bore and finish ream a hole through the center .001-.002 greater than finished boolit diameter, deeper than .38", .50" will do nicely. Cut the end off the stock .25 -.38" long. Chuck it up and smoothly radius the hole edges (.06 is plenty) for an easy start for the boolit on both sides. I used a burr knife and polished with 400 grit sand paper. You now have a very short "partial sizing die ring" with a hole radiused on both ends.
2. Chuck up and counterbore the face of the Lyman die you plan to size with to .445 dia x .010 deep concentric within .003 with the sizer hole. I used .006 soft copper shim stock and this face will house the GC material in disk form centered on the sizer hole. Then, radius the entry hole on your sizer die (.010 approx), so tthe GC material will flow around it without being stretched much by going over an abrupt corner. How much is a matter of experimentation. It should provide enough resistance to smooth the shim stock onto the heel of the boolit without pleats and wrinkles, but not so much that the heel tears through it.
3. Buy or make a 7/16 punch to punch disks out of the copper stock. I punched them against smooth end grain wood which was ash, I think. I expect harder woods will be better for this. Lead would also work. The disk edges should not have a pronounced radius, since the counterbore on the sizer is only .010 deep and they would be hard to center. Whatever wire edge they do have should face upward so they'll become ironed into the heel band and help hold the check on.
Install the sizer in the 450. The way it works is put the copper disk in the counterbore of the sizing die, put the sizer ring on top and put the boolit heel in the hole and push the boolit through the ring and into the sizing die in one smooth motion, lube and eject. Remove the ring, center another disk on the sizer, replace the ring and you're ready to go again. The downward force exerted by forcing the boolit through the ring is to hold the ring against the sizer, keeping the disk flat rather than allowing it to bunch up into a "cupcake paper" and be wrinkly around the boolit heel. Depending on the as-cast dia, sometimes they did wrinkle up a bit, but I could tell no difference in performance. I also tried this with aluminum pop can material, but found that it needed to be annealed to result in a clean corner at the heel. This can be done with a torch, but I really didn't want any extra aluminum oxide wearing on my bore. I wanted to try some 7-UP cans, which used to be steel, but they had switched to aluminum by then. Might've been hard on the punch as well. Using .4375 dia copper disks will work well for .30 through the various 8mm's. These are a good bit cheaper than conventional GCs and can use a number of different materials. There's no GC-maker to build or buy, since the boolit itself accomplishes that. For those with access to the tooling it's a simple thing to do and isn't one of those 10-minute jobs that takes three days.