I believe the 69's came with several engine options. 230ci, 292ci L6, 283,307, 350, 396 ci V8. If you have the 292, there are several things that are bolt on that can give you 80-100 RWHP. If you have the thriftmaster 230, it will run like it has asthma no matter what you do.
All of the SBC engines were underbuilt, and are a great foundation for low buck performance upgrades.
The type of transmission will affect how well your vehicle tows. A well maintained Automatic, IMO works the best. The reason being is that you get Torque Multiplication with an automatic on the order of X1.5.
With that said, Engine/Trans/Rear Ratio have to be considered as a "package". You cant stuff a wild cam in it and expect a miracle. The most dramatic change you can do without breaking the bank is to change the rear axle ratio by installing a new Ring and Pinion Gearset. You would have to go with a numerically higher (lower ratio)gear to achieve breakaway torque, and desired sustaining torque. Sounds complicated but it's not too tough. Let us know what you are running, Eng, Trans, Rear Ratio, 2/4WD, Coil Spring or Leaf Spring, and the weight of your AS, and I will work out a couple of options for you that will have the power that you need.
If you are interested, I replaced the vacuum brake system on my GMC Sprint to the Hydro boost system. PM me and I will send you the pics and parts that you will need.
That car can bury the speedo (120) in 15 seconds, and it use to be a terror to stop. Not any more.
I think it's awesome that you want to tow with an older Surbuban. Just want to see you happy AND safe.