Transmission swap

My truck can barely tow this thing. The trailer weighs 5000lbs and I think it might be a clogged catalytic converter, but I would also just like more towing power. What’s some good transmissions to swap in that can tow but also get up to highway speeds comfortably? 88 gmc k2500

My truck can barely tow this thing. The trailer weighs 5000lbs and I think it might be a clogged catalytic converter, but I would also just like more towing power. What’s some good transmissions to swap in that can tow but also get up to highway speeds comfortably? 88 gmc k2500