Old letters of rec / math prereqs

[thecommons]

I'm applying now to various MPP programs, and I have two letter writers lined up, one my undergrad thesis advisor, the other a project manager I worked with for 5 years in consulting. I am trying to think of someone for the third, and I'm trying to decide whether I should ask the instructor of an economics course I took at night last spring, or to try and use a couple of letters I know I had written for me back in senior year, and are on file at the college. Neither are ideal of course, but which do you think is better? And if it's the latter, what do I do if it's a school that doesn't accept paper letters?

Also, most of the programs require taking an advanced calculus or statistics course. I took two semesters of calc in freshman year, but did horribly. I have several upper level physics courses I took and did a little better in, but nothing spectacular. When I took my GREs 6 years ago, I got 690Q/780V/5.5AW, but had to take it again and didn't do nearly as well (rustier than I thought), 660Q/690V/5.5AW. Because of my less than stellar math grades in undergrad, is it worth retaking the GRE to try and give a better impression of my math ability?

Thanks.