By adding anything to the front of the shaft, it will not become stiffer. Adding length will make any shaft weaker in spine, not stiffer. This already is a 0.300 deflection shaft (the stiffest available). Equivalent wood spine is 104#.
Even when you pull the Bob Lee to 31" you hold about 66-68# at 31".Your problem is, that this arrowshaft only weighs about 11 grain/inch, so your full length arrow is only about 350 grain+50 grain (nock+fletching).
Spine wise, you're about 15# too heavy, assuming 66#@31" would require an approximate shaft spine of: 56+15+15=86# with a 125 grain tip. Assuming you use 175 grains as a tip weight,your arrow will be about 575 grains. So there is about 100 grains missing. Tip:
1.) Fletch with 60/120 4-fletch with 4" fethers 2.) Fill the shaft with braided nylon rope up to 8" to the nock end. This adds with a 7 grain/inch rope about about 160 grain weight. It will weaken the spine slightly, so you may loose 25 grain point weight.
Should still deliver a min. 700 grain arrow and about 10+ grains/lb.
3.) glue a thin! leather piece under your strike plate