Patrick,
I've turned several bowls on my Shopsmith and I don't believe you'll need as much weight as you think. I used to use about 100 lb. I found two 50 lb. bags of gravel and would put one at each end of the upper ways and then turn. When I put the weight on the lower portion of the unit (lower ways or on the legs) I found that the upper ways flexed and moved enough to offset quite a lot of the gain in weight.
One other way you can do it is to make a simple box that will hang from the ways and fill it with something heavy (lead weights, concrete, etc...).
Final note: if you are going to do a lot of turning on the Shopsmith, you'll find that the main shaft will have to be replaced every year or two, depending on the amount you do. I've had to replace mine twice.:BangHead:
Good luck with it and post pictures.
Jim