I am missing the spring between the input yoke of the upper housing and the trans, part # 36 in the picture. I dont remember it being there when I pulled the drives off before. What is its function and is it important to have in back in there?

What transmission do you have? Borg Warner or Merc Trans ll. That spring is only used with the Merc Trans ll. B-W has a male splined input shaft and the Merc Trans ll has a female splined input shaft.

i beleive it is only to make it easier to put the drive back on, thats what ive heard. If you do have a merctrans make sure that it did not fall off inside the tailstock and is sitting there waiting to ruin your day

we just put an engine in a fountain today that has #4 drives and a huber 1200 trans,it has the spring,it helps to line up the drive input shaft into the tailstock.

The trans is a Borg Warner trans. So I am guessing that it not being in there is ok, it may just be a pita to get the yoke engaged.

No quessing. Deffinately not used on B-W's. Should be the 2 o-rings though parts 36A. You can see the part I.D. for the numbers for each trans in the diagram here.
http://www.perfprotech.com/store/ass...LY,986-50.aspx