Hi Spiner, welcome to TradGang! For the future, equipment related questions are best answered in Powwow. A moderator might move your post over there, so you know where to look if it disappears from here.
Howard Hill longbows are not cut past center, so they require a weaker spined arrow than a bow that is cut past center, so that the arrow can bend sufficiently in paradox. For an aluminum arrow, a 1916 would be a good place to start. You might want to leave it full length for a while, unless you get some indication later that it is too weak with whatever point weight you decide to use. Glue in your inserts with hot melt, so they will be easy to remove later if you decide you want to cut the arrow shorter. Try them out with 145 grain points or 150 grain broadheads.
With carbon arrows, a .500 spine would be a good place to start, full length with a similar point. You can also remove the insert from a carbon arrow later on if you want to cut it down, but you have to be careful to only heat the point, and not more than necessary to loosen the glue.
The Howard Hill longbows like a heavy arrow, so I would recommend no less than 10 grains per pound, which would be an arrow weight of 450 grains or more for your setup.