Dave,
Flat tops are the way to go for soldered packs. I don't see any reason that spiral-wound nimh cells of flat or button tops should react to soldering differently but the joints just seem more stable with flat tops.
I would not try to solder GP3700's as my first pack. Honestly, get yourself a bunch of dead alkie cells, Ds or Cs and practice on those. Believe me when I say it takes quite a few practice joints before you get the technique down. Also, if you haven't done so, I'd recommend you search out and read Jim Sexton's (js) excellent post on pack soldering. I forget what forum it's in but it is essential reading.
One of the most important things to do well is surface prep of the cell pads. The most difficult skill to master is bringing the cells together with just enough force and speed to create a thin, solid bond without splashing the solder.
There are lots of nuances to the operation. Do you peel the cell wrap and re-shrink them after soldering? How do you set up a pack that's a brick versus a stick? When do you attach connectors? Etc.
Oh, and buy an extra cell. I always buy at least one more than I need. In the extremely rare case that you get a bad GP cell, you'll have a backup ready to go.
Best of luck to you.
Wilkey