I have a Cub Cadet 128 -- a manually geared mower built around the same era as your 149, with a Kohler 301-A engine (isn't that the same engine that your 149 has?).
Anyhow, despite the fact that the engine could use a good rebuild job, the mower still seems to have more power to tow than the John Deere built riding mower (also with a Kohler engine) that I bought two years ago from Home Depot. (We won't even talk about how it would compare with one of those MTD built Cub Cadets of today -- which are not even as well built as the John Deere's that Home Depot sells.)
The old Cub is really that powerful!
The local Cub Cadet dealer (who has sold and serviced Cubs in this area for over 30 years) tells me that these old cubs were used for many years by the local airport to tow private planes and crop dusters from one part of the airport to the other when they needed to be moved.
He tells me that they do not build the garden tractors now, the way that International Harvester built them back in the 1970's and very early 1980's, and the local airport now has to use full sized CUTs to do what the little Cub Cadet garden tractors used to do.
Also, the Cub Cadet frames of the 1970's -- including the 149's frame -- were built out of iron, not the flimsy thin aluminum or plastic frames of today's garden tractors and riding mowers. As a result, they are in great demand by garden tractor pullers. The frames of today will crush like an accordion if asked to pull the average weight that is pulled in these competitions, but not one of those old Cubs. If those old Cub frames can tow the heavy weights of a pulling competition, I would think they will hold their own (at least compared to the MTD built garden tractor frames of today) on your hills.
So don't sell your 149 short. I know it's an automatic tranny (which means it might be a bit easier to use, but will not be quite as strong as the manuals). But even the automatic trannies of that era ran with Clydesdale horses under the hood, instead of the miniature ponies that run under the hood of the MTD built mowers of today.