Sky, just make sure that you change the pickup screen when you do the pump too, just to eliminate the possibility of junk from the old screen flying up into the new pump.
A lot of times, jumping on it when the engine is still cold will make for more strain on the pump. The reason being that the oil isn't yet warm enough to thin out, even though there's viscosity indexers in it.
The other theory is the distributor shaft could be deflecting enough to make it try to bend & turn like a speedometer cable. SNAP! Clearancing the place where the distributor gear rides is suggested by a lot of the 5.0 books. Another thing the books suggest is slight chamfering of the casting and rotors of the pump. I'm doing this right now on my Melling HV, to try and avoid this type of thing. Just my .02
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Capri306, Moderator, The Mustang Works Online
1979 Mercury Capri, 5.0L -- C4 -- 2.73
1987 Mustang LX Notch