michiganroadkill-
One can modify 284 brass to make 450B cases, but the project can be complicated depending on tolerances in your rifle.
1) 450B brass and 284 brass triumed to 1.700" have different internal dimensions. The 450B brass is thinner at the mouth than trimmed 284 brass. The 450B case is not tapered internally for some distance from the mouth toward the head. The result of this is that trying to seat a bullet in the trimmed 284 case causes a considerable bulge at the mouth end of the case. This may cause problems in chambering depending on dimensions of your chamber. A bulged case may wedge itself in your chamber if it's on the small side of allowable diameter.
The bulging problem can be circumvented by obtaining a custom reamer that thins the trimmed 284 case walls.
2) Again depending on your firearm, rim dimension differences can be a problem. When the cartridge was introduced, Hornady cut the extractor grooves on their cases deeper than SAAMI spec, and bolts on the Bushmaster rifles were made to fit Hornady cases. Hornady's objective is rumored to have been preventing use of trimmed 284 cases in 450B rifles.
The conversion of 284 cases to 450B has been discussed extensively years on the 450B forum. Here's a link to a thread to start:
Turning .284 brass into 450 brass?.
--Bob