Rear Leaf Spring Dilemma

I recently installed the Rustys 3" add a leaf. The problem I ran into was the bolt they sent for the rear leaf springs was too large for the factory leafs. It fit their leaf, but not the factory ones. I contacted them and they gladly sent a smaller one that will fit, however, I am having trouble figuring out how to get the bolt tight on it. It has a circular head instead of a hex and it is impossible to hold on to while tightening the bolt. Maybe there is a simple solution that I am missing, and maybe you guys have encountered this and know how it is resolved. Thanks.
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i used a pair of vice grips to clamp on

that looks to me like a center pin bolt [hopefully thats what you are referring to]

that bolt goes into the circular hole on the spring perch and keeps the leaves centered. you'll probably have to hammer it into the hole and then tightening it will be no big deal.

if a BFH doesnt work, and after you got it in, it doesnt stay tight, maybe some JB weld... but id be wary of using that stuff.

in all honesty, that bolt isnt the greatest necessity. of course it DOES help.
put it in the springs, grab onto the round part with vice grips, tighten it down, then install. the u bolts and plate are what really keeps your axle in the right position. even with that just SITTING in there, you will be fairly ok.

it just serves to keep your springs completely centered, [front to back]
which IMO is a pretty rare occurance [by any measurable means anyway]

Sounds good to me. Thanks for all your help. I should have it tackled today. I started trimming some fenders a little yesterday. Who ever said one job at a time right?

I finally got time to work on it, and it went in with no problem. Somehow when I tightened it a little, it must've had teeth cause it just tightened right up! Now I'm finished with the slight fender trimming and just have to get the flares back on. Think I might hit a trail or two before putting them back on though. Thanks for your help.