need advice on pulling with my 72

[iluvmy72]

My wife thre the wrench intot he gears today when she said she wanted to get a 5th wheel instead of a regular travel trailer......I've tried to change her mind but she's set.....I realise in order for me to do this I might have to make a few changes on my 72 GMC longbed.....I have a very strong Rocket Olds 350(2bolt main), Rebuilt 700R4 with performance shift kit and a 3/4 ton rear end with a 3.73 gear ratio. My question is can I get away with the engine block and tranny I currently have?.....I do have a regular chevy 350 engine and a rebuilt Chevy TH350 with a added shift kit as well but I don't see this adding anything I would need.....But I'm wondering if I currently have enough juice to pull and 32-36 foot trailer that weighs 12000-14000 pounds.....I'll need it for mountains.....if I had money, I'd add an Olds 455 engine and a overdrive add on to the TH350.....what else could I do to boost power with what I currently have?