Hello,
The D2 steel will work for your trem plate, it will have to be hardened after you shape it.
Also the 1045, 1095, 5160, 4140/4142, W1, A2, 01, will work. some are easier to machine and harden than others.
Whatever steel you chose you should google it to see how hard it will be to work and harden.
on this site you can find lots of information about the various metals:
http://www.matweb.com/index.asp?ckck=1
on this site you can find Anchor and stud sets (pivot post):
http://www.axcessories.com/products.asp?cat=116&pg=2
good luck