[clay68c10]

Two things I can think of

1. How much is your rpm dropping when you go from p/n to d/r? If the engine slows, so does the water pump. Might kick up your idle and see if it changes the result. The trans front pump is pumping fluid thru the cooler no matter what gear you select, so that’s probably a non issue.

2. If you are running Vortec heads, they have a different cooling passage setup compared to non Vortec heads. I don’t remember exactly, but it has something to do with the water pump bypass and how that is routed thru the block. If those heads are used on an old style block you have to accommodate for that. Might be worth a bit of research to ensure you aren’t chasing that.