Since you mentioned that you would consider a manual, you might want to look strongly into swapping in a SM465 4 speed manual in it. The SM465 is a very tough transmission and cheap on the used market. The advantage of this transmission over the TH400 is that there is no torque converter slippage so you put more power to the ground and with a 8000 lb load behind a 135 hp 6.2 diesel, you're going to need as much advantage as possible. Also, you won't have to worry about the transmission heating up. The TH400 is notorious for building heat due to the lack of a lock up torque converter. However, the installation will be a bit more complicated since you'll have to install the clutch pedals and cut a hole in the tunnel for the shifter.
Also, I notice that your Suburban is a K10 1/2 ton and you're wanting to pull 8000 lbs. I've even done a little more than that with my 6.2 Suburban but that's a pretty heavy load for that truck and it's going to be very slow on the road behind any 6.2 diesel. What gearing are you running? I hope it's not 3.08's.
1990 ¾ ton 4x4 Chevy Suburban
-Cummins Diesel - 12 valve - factory rebuilt
-6 speed bullet proof manual transmission - NV5600
-Gear Vendors Overdrive
-Upgraded Holset HX-35 turbo
-NP205 iron transfer case
-3.73 gears
1982 ½ ton Chevy Suburban
-6.2L diesel - high nickle crack free 1982 block
-Stans headers
-Ported heads
-Timing gear
-4 speed automatic
-3.08 gears
-30 mpg on freeway