The "chute" type is the kind you already have (sometime also referred to as "fairlead"). Like a cattle chute - raised sides that guide the cattle through an area, not down into a hole. Now, it could be that the name has been misused in your area for so long that everyone around there just calls it by that.
Although I think that even the existing anchor roller is stronger than most would give it credit, including the pulpit being plenty strong... I agree with Half - use a cleat. A cleat is a much more robust anchor point for the retrieval. Also as Half mentioned, the midship cleat is a good idea, too. It's just a better attachment point, all around, for this job. I suppose an aft cleat would be at least as good, too.
As far as just "anchoring" goes, the actual depth makes little difference as to the load affected on the pulpit/roller. 450' of scope... I'm not sure what you mean there. Scope is a ratio of rode to depth. The more "scope" you have, the better - not the other way around. There are ZERO issues with anchoring where you do using your existing roller. I won't argue that there is LESS load put on the pulpit when using a platform mount since it's just plain physics that says there is, indeed, less stress. But that doesn't mean that it's a PROBLEM to use your existing roller setup. Also, look at it this way... boats much larger than yours use the same setup as you currently have and they don't have issues. And their platform isn't necessarily much bigger (some are the same exact size), especially considered to how much more their boat weighs.
If you still want to move forward, though...
-- You need to address rebuilding/fiberglassing the hole you cut out. Keep in mind if you don't make it look nice, it could affect the resale down the road (first impressions)
-- If you can build the darn thing yourself, what's the hold-up? Get 'er done. If you're making it yourself, design it around whatever size roller you want. Probably 2-1/2" to 3" for a small boat. But keep in mind that you don't have to do it the "Grady way" - their way is just one way of doing it. If you're confident in your manufacturing skills, then just figure it out and make it work. It's not rocket science.
-- Download the parts manual for a Gulfstream of the appropriate year. I imagine that should give the size of a replacement roller.
-- Google for "platform" or "plank" mount anchor roller. See what you find. I saw one by Anchorlift by using those terms.