A 78 should still have a bolt in trans cover so you could always find a new one to cut up. You would still have to cut the carpet either way. The NV4500 is always an option. If you look around you can find one cheap. It too is a 0.73 overdrive. I do think heavy towing will suffer with an a833. It may work for your needs, so on your current trans try towing with out using first. The a833 has a 3.09 first gear the sm465 has a 3.58 second gear. So it should give you some idea, but a sm465 second gear is still 0.49 lower. If your look for more here is a good thread on it.
"What can be said about the 4-speed New Process NV833 overdrive manual transmission?" Also down below is that same chart with the a833 ratios plugged in.
