Just got back from two week RV trip with good news and bad news. Good = The new tranny cooler worked great. Most of the time the temp ran from 150 to 165. One day it was over 100 degrees all day in Eastern Washington, plus had a headwind. The engine never overheated although the clutch fan came on quite often, had the AC on all day too, doesnt' seem to affect the power on the 383 (the cooling system on this Suburban is fantastic). The trans temp stayed at 180 all day, never went over even on hills. Eastern WA Route 28 is up and down a lot, no cut and fill on that highway, almost as bad as Missouri.
Bad news, cracked another exhaust manifold (4th one), probably when climbing long steep hill east of Orondo on Highway 2. Kept it at 3700 rpm and 50-55 mph. I thought the 383 wouldn't do that since it has so much more power and didn't have to work so hard getting up the hill. Probably the fact that weather temp was over 80 that day. Even tho the DW will not like it, I'm getting headers, probably $400 shorties from Summit. I am not going to slow down on hills, although always keep it around 65 mph for tire safety sake.
Thanks for the info on the torque converter operation. I'm only using 4th on long downhills now. Any level or uphill always pull it down to 3rd.