transmission problem?

[cali_surfer]

I'm posting this for a buddy of mine with a 70 GMC. He has a 454/Turbo 400 combination in his truck. The truck sat for several years on the side of his house undriven. The project started with an engine rebuild but things don't always happen the way we like. Anyways, the transmission was left in the truck and the torque converter was removed, stored flat and sealed off. Fast forward to now. The truck is back on the road and is quick the terrorizer. The 454 gets up and runs. However, he ran into a problem recently with his transmission. There is a really big mountain near by, he has friends that live on the other side. When he goes up the grade (its about 6% for 3-4 miles) the thing starts smoking. Transmission fluid is leaking out onto the header collector on the passenger side. What's weird is that he has no problems with this back down in the valley where its flat. His trans pan leaks some, and he replaced the input seal. The converter and flex plate appear to dry and free of any oil. He's kinda baffled as to what is causing the fluid to gush out and start smoking.