The r/box bolts are same for iron and alloy heads, 4 of 5/16" x 3-1/2" CEI (BSC or Cycle thread) and 1 of 1-1/2". The studs differ, iron head 4 of 5/16"-18-26 x 1-5/16" BSW/CEI, alloy head 4 of 5/16"-18-26 x 1-13/16" BSW/CEI. The front studs on the plunger are 1-7/16" long.
3/8" BSF is correct for head bolts.
Cheers
Bob got in front.