Bob - Yes, I know where you can get the proper kit, if you have the tag number of your present carburetor. You have found one of the reasons why we started producing kits years ago, when we were doing restorations only, and getting incorrect parts at the parts stores. The correct pump for the 1958 261 is part number 7002091 and has the flat shaft.
Accelerator pumps come with both neopreme and leather cups (the leather is MUCH superior, but the neopreme is MUCH cheaper).
Often the neopreme cup can be easily removed from one pump and placed on another. Check the diameter of the "thimble" holding the pump.
The leather pump is more difficult to transfer, but it can be done. To do so, very carefully grind off the peened ends of both pumps; and remove the thimble assemble (thimble, leather cup, and expander spring). Now mill approximately 0.050 (50 thousandths) from the thimble stop on the shaft of the old pump. Assemble the new leather cup and spring with the old thimble back on the old pump. The 0.050 will allow you to repeen the shaft over the thimble. The pump should not be 0.050 shorter. This will cause a slightly larger shot of gasoline when the pump is used, and can slightly impact your fuel economy in city driving.
It might be easier to acquire a "junk" Rochester with the plunger for the round shaft, and switch them.
Jon.
