Birch makes fine heavy arrows. I use it for most of my heavy arrows. Ash, oak and poplar are also good materials.
Do not use an arrow <50 grams for a 120# bow, IMHO. The risk is in breaking the arrow if it is too light. Devastating result. Yes, straight grain is very important.
No, I don't spine any of my heavy arrows. I believe it is completely irrelevant to heavy bow flight shooting. For EWBS and CWBS shoots, it's all about the arrow weight.
Standard arrows, 3/8", min 31.5", must be 52 grams.
Livery arrow, 1/2" min 30.5" , must be 64 grams.
1/4 # arrow, 1/2" min (113 grams), min 30.5".
Check
www.englishwarbowsociety.com and under 'articles' it will give you all the details and specs.
No, I don't shoot any parallel shafts, especially 1/2". I taper them to 3/8" at the nock. If you want to see how I do that, do a search under this forum heading for 'tapering war arrows.'