WOW, I have a similar set-up and the exact same problems. An 83 1 ton Chevy dually with a tired 454 (nagging exhaust manifold leak) pulling an 86 42ft Chapparel. With both cars in I think I'm loaded at 13,000 lb.s I have considered a 454 HO crate package from GM Performance or just rebuilding the stock motor into a stump pulling tq. monster. But with interest rates in the dirt, you can sure make a case for driving home a new truck.
Please post how you resolved your situation.