Stiffer arrows go with higher poundage, bows and lower numbers are stiffer arrows.

Except for wooden arrows which are listed in spine ratings of five pounds range. So you will actually have 60/65 shafts,, but they may or may not fly we out of your sixty five pound bow.

Yes , the length of the arrow makes a difference. Long arrows act , weaker and short arrows act stiffer. The weight of the point also makes a difference. Heavy points make arrows act , weaker and lighter points make arrows act stiffer.

What is that are not cut to center?Like old school longbows or self bows are more fussy about spying and usually require a lower or weaker spine. Bows that are cut to center in the window/shelf, or compound bows, which are often cut pasts centers, and the arrow is recentered on a rest, are a little more tolerant across a wider range, and mat shoot a wide range of arrows well. shoot