Kevin,
You may like to consider whether you need a new cross slide at all. I contemplated a similar modification to my Asian 9 x 20 lathe some while ago. Even purchased a secondhand Myford cross slide in preparation for it. I got a fair bit through the mods when I sat back and re-appraised the situation. Comparing the two cross slides, I realised that there was so little difference in the overall dimensions, that I could probably, quite safely, mill some tee slots in the original cross slide, thus avoiding much butchery of the topslide. Long story short, I did ( with much trepidation, mind you ) and the end result I find entirely satisfactory. Obviously, your cross slide may not be up to it, though I very much doubt that. Only you can decide. I am not familiar with the Boxford, though I suspect, if you are contemplating a Myford part to replace the existing one, it must be of similar size. Good luck. Here's a couple of photos to give you some idea of how it turned out – from my album ( Cross slide Modifications )
Before:

and after:

Best of luck,
John