BW460-
Yours is not a stupid question at all. I'll make three points in this post for you to consider. My advance apologies for wordiness.
[POINT #1]I used a Wilson trimmer for a couple of decades, though I no longer have it. I'm not sure that the 450B case will fit the 284 holder.
From the L.E. Wilson blurb about the trimmer: "Wilson case trimmers are unique in that the case is held by a case holder which simulates a rifle chamber. Cases are held in case holders by their own body taper." Here's an image from the Wilson web site:
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As you know, the case head and rim diameters are irrelevant to the positioning of the case on the trimmer frame. What is important is that the taper of the case has to match the internal taper of the case holder. Friction between the case and the tapered hole in the holder is what prevents the case from turning when the cutter is applied to the case mouth.
As you also likely know, for some cartridges, taper will differ between fired and sized cases, and different holders are required. For example, with the 308 Winchester, different holders are required for new/sized brass and for fired brass. With the 30-06, firing doesn't change the taper enough to require two different holders, although the case holder will hold fired and sized cases with differing lengths of neck protrusion.
If the SAAMI drawings I have for the 284 and .450B are correct, and if I've operated my calculator properly, the diameter of the 284W case tapers 0.0160 inches per inch of case length. The 450B tapers 0.013333 inches per inch. Both the 450B and 284W cases have the same head diameter of 0.500". My guess is that the 450B case will fit the head end of the Wilson 284W holder just fine. However, the 450B case may not go entirely through the holder, and the mouth my not emerge from the holder so that it can be cut. Even if the case and holder tapers are compatible, the Wilson holder may be set up for the longer 284W case length, and the case mouth may not emerge from the holder.
The answer to this problem of course is to order a special Wilson case holder that costs $22, compared to $14 for an off-the-shelf holder. To make a special-order case holder, Wilson asks you to "send in three cases in the condition in which you intend to trim them (ex. new, fired or sized)".
I think most reloaders of the 450B full-length size their cases in the reloading process, just as in reloading straight-walled pistol cartridges. I've not read or heard of any 450B reloaders who neck size cases. For full-length sizing, you will probably get the best case-to-holder fit for your brass if you send in three fired-and-sized cases when ordering a custom Wilson case holder.
Despite the Wilson trimmer's quality, ease of use, and uniformity of trim, I sold mine because I was reloading a lot of odd cartridges and the expense of ordering custom holders for these was going to be excessive at the time. For reloading a few different cartridges, or if you can afford a bunch of custom holders, Wilson trimmer is great.
[POINT #2]If you read pertinent threads on this forum, you will find that trimming techniques are rarely mentioned. Case length growth is seldom encountered in 450B reloading, just as it is rare in reloading most auto pistol cartridges. Cases length tends to decrease with repeated reloading and firing. You may find trimming unnecessary, so I'd hold off on ordering a custom Wilson holder until you find trimming is needed.
[POINT #3]In some of my experimental work reported in this forum, I've used my bolt rifles with 450B loads at pressures exceeding SAAMI max. With some of these loads, case lengths have increased to the point where trimming was needed.
A couple of decades ago, I moved to the Lee system of trimming. (Lee's ideas are good enough that Lyman has recently begun to market their own version of the Lee system.) For cartridges like the 450B for which Lee does not stock standard case trim length gauges, Lee will make a gauge if you pay them enough. Fortunately, the mechanics of the Lee system are simple, and I have been able to cobble together gauges for some pretty odd cartridges. For the 450B, I used the length gauge for the 222 Rem. That cartridge has the same 1.700" length as the 450B. I drilled a washer with a hole to fit closely the gauge stem, and turned its outside diameter to just under .450" to fit inside the case mouth. The washer holds the case centered on the trimmer cutting blades. Trimming is trouble-free if I hold the case vertically, mouth down.
If I've messed up any explanations here, please comment or ask questions, and I'll try to clarify (or retract, if appropriate).
Good luck.
--Bob
edited to remove a major oopsie.