Next issue in sorting out this cooling system: thermal clutch fan mounting height. I assume fan height is measured from the end of the shaft mounting flange to the fan mounting surface. I'm getting about 2"...pic attached. This is different from the 1.53 or 1.62" info at http://www.buickperformanceclub.com/FanClutchInfo.htm If i'm measuring correctly I could get the fan closer to optimal position by getting a shorter fan.
I would suggest taking the clutch off and having the parts store order one for a 70 455. Take the old one with you and see if they are different. I am guessing yours is wrong. It looks too long. All for 70 and 71 I have seen were the same length.
Then again the 71 long water pump is not meant to be used with a 70 fan shroud. That’s your real problem I bet. That’s at least an inch to long.