I owned a 77 Howard 21 DC for years and loved that boat, last time I saw it, it was still looking good and running strong. My set up was a hyd cam 454 with good oval heads and a Holley / torquer manifold. Turbo-400 trans with a manual valve body, and a 10 degree Casale split case, with 22 gears, pushed by a Tom Black steel 12 X 15 three blade. In my opinion the Powerglide would be my tranny of choice if I were to do another. They are lighter, smaller, much simpler, and require less power to turn than a T-400. The low gear in a T-400 is useless, boat would move so slow, the steering was comprimised. Second is good for idleing in a 5 mph zone, and also if you have a boat load, and a monster on a single, you can pull him out and then click it into high. Park is also useful, as in an emergency you could safely get a skier out of the water, should you have an issue where stopping the engine was not possible, such as a low battery or faulty alternator. If you go with either a Glide or 400, be sure to get a shifter that has a reverse lock out safety feature.
Crane sells some nice hyd roller BBC marine cams, that along with the usual BBC mods would give you the power your looking for, along with good idle and skiing manners.