Jazzato,
If I read you right, you'd cut the case head off with a hacksaw? That may make a difference to Redding when they go to extract that case, since the head needs to be present for the standard stuck case removers to work. All the major makers sell these, and most are pretty similar. They involved drilling out the flash hole and then tapping it. A cup goes over the base of the die and an allen head cap screw passes through that, and threads into the tapped case. from there, the case can be easily backed out with an allen wrench. You need one of these, even if you only use it once or twice. Think of it as the eraser on the end of your reloading pencil.
Don't sell the 550, either, they're great presses. However, they're definately not the first choice for this sort of reloading. Someone else mentioned using it as a single stage press until you get a handle on things, and that's good advice. If at all feasible, I'd recommend getting another press, single stage, probably an O-frame or something like the Forster Co-Ax for laoding the magnums. It'll turn out better ammo and make your life much simpler in the process. As I said, I truly love Dillon presses, and they have some of the best customer service in the industry. And yes, you absolutely can reload belted magnums in progressive presses . . . but they're really not the best choice for this sort of thing. For what it's worth, I load rifle ammo, even some short range match ammo, on both Dillon 550s and a 1050. However, they cases I start with have already been sized (on a single stage press), cleaned thoroughly, trimmed and inspected before they ever hit the Dillons. In essence, all I'm doing on the Dillons is primer pocket swaging (in 5.56mms, anyway) repriming, powder charging and bullet seating. Not exactly a "true" progressive operation as Dillon might envision it, but it's a process that works well for my needs.
The Dillon powder measures work great for many powders, not so hot for extruded tubular powders. I normally use a Redding BR measure on my Dillons when loading rifle ammo, especially with stick powders. They sell an adapter to do this, though it is a manual operation to throw the lever (which I like anyway). As has already been mentioned here, all this is irrelevant if we're talking long range match ammo; you'll want to weigh that to get the best accuracy.
There's a lot more here, but you'll bump into all that in due time.
Hope this helps,
Kevin Thomas
Lapua USA