I can tell you that a 39/21 is pushing it for me on those kind of grades i've got a similar but a little more complicated delima with the gearing on my current Project. I can make 2 of the 3 hills in my neighborhood over 6% with the combo but i've got one that is between 8% and 10% that i am out of the seat and on the pedals the whole time and i think i would be moving faster if i was walking.

My complications are from the fortunate/unfortunate problem that the wheelset that came on my bike purchase were very high end from the early 90's and unfortunately they weren't shimano/campagnolo hubs they are suntour superbe pro (8 speed cassette)

I think you would be much happier and most likely more inclined to ride more with the 39/28 climbing combo. look at it like this after you ride some and get back into better shape you can always just offload that cassette on the used market and swap back to the other. For the minimal investment required for the wider range cassette it's almost a no brainer in my book.

You might also try a nearby co-op where they are much more likely to be helpful with not only finding the right cassette(which they may have for less than new prices) but also helping you change it out.