I'm with Matt. Even a 782 would be ideal. I know from good experience with both that the 782 will outperform the Cub Lo-Boys. My 782 has a 50" deck and is quicker than my 67 Lo-Boy with 60" deck. Plus the 782 is very compact, plenty of power, very maneuverable, and relatively inexpensive for what it's capable of.
If you are set on building a bigger tractor, perhaps a Super Garden Tractor hydro rear end? They're at least gear reduced a little bit compared to older rear ends (which I just recently learned myself).
Just a few ideas
