I only suggest you test this. Wether you convert to 720 24p or DVCPRO 50 24p is not really the point, but to get it to a conformable p format befor mpeg2 encoding is.
Thus:
Your sugestion may be much better than mine!
(Scratching my head a bit...) Probably is actually!
Try!
And then encode 24p mpeg2
What I hope (but dunno) you'll see is some improved resolution.
Final Cut> Export Using Compresor> Advanced Conversion>DV50 NTSC 29.97> Progressive 16:9>Frame Controls off. I have not tried on, but will over night, I will set everything to best.