I think the reason why you won't find anything is because angle iron won't withstand the twisting forces that the hillers will put on them.
I'd recommend replacing one of the angles with square tube; you can then mount the hiller shanks to it with u-bolts and a small plate or any of the similarly conventional manners that I'm sure you've already seen.
I'd recommend replacing one of the angles with square tube; you can then mount the hiller shanks to it with u-bolts and a small plate or any of the similarly conventional manners that I'm sure you've already seen.