I ran a very similar Myford Speed 10 for 15 years and used a similar double-swivel vertical slide. The crossslide on these machines is pretty narrow and support for milling is limited. Check the gibs aren't slack. Lock all slides except the one you're feeding for the cut. If you have a leadscrew clutch, disconnect it and close the halfnuts so you can control depth of cut on the leadscrew handwheel, and lock the saddle for each cut.
Cut slow, with sharp tools, and cut light, and you could get a lot of useful service. I still have my Myford vertical slide, but I now use it on a Warco WM250V lathe, which is larger and more substantial.
Well, that's my two-penn'orth…