I think you can get a 4-wheel 1000 lb rated engine stand for about $50. But it wouldn't have the rotator. You might have that much in modifying the base of yours if you had to buy the materials, But it wouldn't be that much effort.
I've modified two engine stands into a cab and frame rotisserie, so I don't think you're crazy for modifying that 3 legged stand to be safer.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.