Part of this depends on the position of the cam. If the excentric lobe that the push rod rides on is "out" then you will be compressing the fuel pump arm a fair amount as you tighten the pump down. If the excentric lobe is "in", then you will have much less compression as you tighten the fuel pump. The aim is to hold the push rod in place until the fuel pump arm can contact it so that it can hold it in place. There should be no "slop" when the pump is in and tight.
I replaced the fuel pump on my 69 LeBaron 2 years ago and ergonomically, this was one of the most unpleasant repairs that I have done. It was right up there with the rear plugs on a 92 Grand Cravan and the clutch master cylinder on a 90 Isuzu PU.
Good luck.
