727 shift probs

I was working on a 340 Duster overheating problem (fixed by replacing water pump and new fan clutch) and in the process I replaced my radiator, thinking it was clogged. The new replacement radiator looks identical, has the same locations for the transmission lines, so I swapped the fittings, hooked the tranny lines back up, topped of the trans fluid. However, I used Dexron III / Mercon, instead of the type F that was originally in the 727 (called prev owner, found out about the type F, too late)

Before working on the overheating prob, transmission shifted perfectly.

Now, the transmission at times revs up a lot higher in 1st before shifting to second and slips when shifting to second. I sometimes have to let off the gas pedal to get it to shift.

So far I have pulled the trans cooling lines of the new radiator, blew air through the radiator to verify it wasn't plugged, dropped the trans pan, blew air back through the lines back to the tranny to verify the lines were not plugged. I went a head and replaced the filter, put on a new pan with a drain plug. I have since drained all the fluid (converter has no drain plug) and replenished w type F, still have intermittent slippage. 727 does have a shift kit in it. When working right it does shift hard. There was very little, fine metal accumulated on the magnet on the pan, 'normal wear' according to those I have talked to. I have driven the car several miles, making sure there are no air pockets and the fluid level is correct.

I am planning on one or two more 'flushes' with type F.

Anyone have any suggestions?