Adude,
I've been having the same issue. I'm shoot a 29" (400's) arrow with the 50 grain inserts, 125 bh. They shoot perfect out of my 55lb Montana longbow, but out of my 55lb Hoyt Dorado they seem a little weak. I draw right at 28".
I'm going to buy a test kit of different spines to test, because I would like to have more versatility as to which bh's I can use. Right now I'm completely limited to heads that come in 125 grains.
If you get it figured out, you might post what you ended up doing.