I did a little research on frame geometry, and the numbers show that two of my bikes are pretty darn similar. My 1987 Trek 520 and my 2010 Surly Cross-Check stack up like this:
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| A | B | C | D | E | F | G | H | |
| Trek | 64.7 | 72.7 | 72 | 58.9 | 45.5 | 5.2 | 6.5 | 108.1 |
| CC | 64.5 | 72 | 72 | 61.0 | 42.5 | 4.4 | 6.6 | 105.5 |
The cross-check has shorter chainstays, and a longer top tube. The Cross-check also takes monster truck tires, while the Trek tops out around 38mm. I’m thinking a drivetrain switcheroo might be in order. The CC could easily be my primary bike, but it isn’t because the Sturmey Archer gearing is a little too high for most of the hills around here.
With a wide gear range, and suitable tires, the CC might be the better bike for 99% of all the riding I do, and the Karate Monkey would probably be better for the other 1%.
Not sure what I’d do with my beloved Trek, then.
Decisions, Decisions.
If you put the derailer system on the CC, you still have a fixed rear wheel and an IG wheel set. It sounds like you need another front wheel and a new frame.