Three-eights inch diameter, 16 threads per inch National Coarse, 3/4 inch long hex head bolt.
When changing fuel pump on the side of the road, and you have no bolt in toolbox, use the pass side bottom water pump bolt for locking the pump rod. Finger tight, then do the swapout, replace bolts.
I'll probably catch alot of flack on this choice of bolt, but, if I remember correctly, no coolant came out. Maybe I didn't have coolant in it at the time..........so long ago.