David, if you want to do it then you need to buy the entire package from Intermeccanica. I spent months researching and looking for these things and it won't be easy without IM's help.
280 a piece is slightly more then Stoddards, but Stoddards won't order them or stock them until you order them, so it is pretty close.
Also you need to know that the original bars are impossible to find in good shape/useable shape. They were 28mm in diameter. Where they attach to the bumperette needs to be an original bumperette to use an original tube and it needs to be the overrider tube style bumperette, they are different. Where as a reproduction bumperette even from Stoddards is going to be for 25mm tube. Henry's tubes are 25mm and awesome.
What you need to understand from what people are saying is that the frame work holding your vintage bumper may be in the way of your new bumperettes.
If you really want them, I would call Henry and ask him if you send the bumpers to him if he would be willing to fit them for you. Yes it will cost you extra, but Henry's expertise is worth it!!!
Please email me if you have further questions as I have researched and searched these to death and can answer any other questions you may have.
Below is a picture of my 56 original single rear bar, don't even think about trying to find one of these....impossible!!!