Pete,
As long as the frame the wooden bench top is attached to is reasonably rigid and does not develop a rhythm at any of the lathe speeds, it should work just as well as an original Southbend cabinet base. Actually I would expect an advantage to the wooden bench top as it will help deaden vibrations...
Many years ago when I was in Alaska, I worked part time in a contractors maintainence shop doing machine work such as making shaft adapters and replacing bronze bearings. In that shop were two lathes, a 20" LeBlond and a 12" Atlas. The 12" Atlas was a light weight bench lathe mounted on a bench made of 2x4 and 2x6 construction lumber and was an amazingly nice running little lathe. The LeBlond had noisy bearings and was not good for small work and that little light weight Atlas on the wooden bench was the shop workhorse for anything that we could fit in it... Years ago, after coming back from Alaska, (1969) I worked in an oceanographic instrument shop for a couple years with several 10" Southbend toolroom lathes on the cabinet bases and frankly, I see no difference... Only advantage to the cabinet base is keeping the lower unit enclosed and protected from chips etc. To my way of thinking, the only real advantage is in the cast iron pedestal bases of the bigger lathes such as the 13" & 16" models due to the mass to absorb vibrations and efficient use of floor space.
Happy Shooting,
Mitch & Shadow...