Will there be any quality difference in these 2 methods for converting SD/720x480/29.97 to ProRes422/1440x1080/60i?

by default, both of these methods produce new 1440x1080/60i clips with the SD scaled to 225%, ie. to fit the frame top to bottom, but with black bars filling the left and ride side of the HD frame – what do I need to to do to scale the SD by ~306% ie. To fill the entire HD frame and crop itself at the top and bottom?

First here are the three methods that most go about this:

1. Allow Final Cut Pro to scale in the sequence (dropping SD content into HD sequence)
2. Process footage inside Final Cut Pro Studio
3. Process and scale footage inside Adobe after effects

In my opinion, Final Cut Pro is not the best application around for scaling. On my last feature I solely used after effects with great results.

So on your question: The easiest option for you will be number one. Simply drop your SD footage into your HD timeline and allow it to resize with two functions.
They are called scale to sequence and conform (Under Sequence Setting Menu).

If you are attempting to do the impossible, putting 4 x 3 into a 16 x 9, this will be the best you can get without further stretching the SD to fill HD without pillar box.

If you have the time go with after effects - you won't be disappointed.
-C