Yup. I can agree. This is what I did for a Halloween quick set-up at my house:
I had to flip the bracket outwards and used regular bolts from my box of "spare lighting parts" through the pre-drilled holes in the T-bar to make it come together. I wanted to use o-clamps, but it wasn't happening. It was a big faster at the time to just use bolts than use the o-clamps.
As far as length is concerned, yes, the mounting bolt is a bit too long for the Mega Bar 50's. I think the idea of these lights is to be floor mounted. As far as the pre-drilled holes are concerned, if you're using o-clamps, then you don't need to bother with those.
I think in general, the T-Bars are pre-drilled to accomodate Par Cans anyways. To me, that makes sense. 4 cans on a tree, evenly spaced. For that it's perfect.
If you want a hardware store solution, I suggest you take a clamp with you, and find matching nuts to replace the wingnuts and buy in sufficient quantity to let you have enough for each o-clamp. It's not going to cost you all that much anyways.
Now, correct me if I'm wrong(and I very well may be), but I think if you want to go traditional T-bar mounting, I think you'll be limited to 1 fixture per side. I don't think you can go up/down due to the space needed by the o-clamp. I forget. As I stated before, I was short on time on my previous set-up(I came up with it like an hour before the beggers go door to door looking for sugar-based goods).