Steve, I'll leave the HP question for others who are more experienced with calculating/estimating that value for you. On the rear gearing, that 700R4 has a 0.70 to one final drive (over drive) ratio, but it also has a first gear ratio of 3.06 to one which makes it a great choice for your truck. The gearing you'll want is going to depend on the rear tires you decide to run, and what you want from the truck. Use the formula Cruise RPM = (mph x Gear Ratio x 336 x OD Ratio)/Tire Diameter to run the numbers, and you can shift the variables around to figure what you want, like Gear Ratio = (Tire Diameter x Cruise RPM)/(mph x 336 x OD Ratio), run your numbers and then pick the closest available ratio that fits and re-run the numbers to get exact. My deuce roadster project has a 700r4, 31.7" tall tires, and with a 3.50 to one gear ratio will be cranking 1818 rpm at 70mph - a great cruise rpm for the 350 ZZ4 engine.

Hope that helps!