Spark Plug Mix Up!

Hi all, I am in need of some expert advice on a slight mix up in spark plugs on my 1999 Taurus SE 3.0 24v. I replaced my plugs the other day when I noticed that 2 of the plugs that the store had given me were PP755. The other 4 were the correct plug, being PP754. From what I understand the last number in a spark plug is the heat range. The diffrence from what I hear being very slight. I guess you can figure out that I noticed this mix up after I change out all the plugs! I talked to a mechanic that did my inspection and he claimed that it wouldnt be a problem. Unfortunately, this is the same mecahnic who failed my Taurus for bad lower ball joints, when In fact the ball joints were fine. (I went to get a second opinion) Can anyone tell me If they think this would be okay to just leave it the way it is? The plugs were Autolite Double Platinums. Or should I go back in and put the correct ones in? Thanks:banghead:

I would base my decision on how it's running.......

Seems to run fine.