Beautiful looking trailer, I wouldn't want to hack it up either.
Is it just going to rest on the 2 sides? or is it going to overhang the front pipe as well?
I see no reason to support it on the deck just go with the 2x pads.
If it's just resting on the 2 sides, instead of a long piece bridging across from pipe to pipe just bolt 2 pieces parallel with the top of the pipes where the bucket will rest. use U-bolts to make it more stable. No drilling the pipes and you don't have to remove them, will have to counter sink for the bolts and nuts so they don't extend above the top of the wood.
A 4x4 cut out on the bottom to mate with and drop down over the pipe a little would be nice.
JB.