You need to match guitar out electrically with analog inputs, that's the reason you need at least a D.I. box.
Then you can have some very cool and interesting processing of your guitar in SFP, with all the no-latency benefits, with the add on that you could monitor your guitar fully effected while recording it clean, and apply that processing on the recorded track, so you can change it if you change idea, arrangement or you want to double it with different sounds...This is one of the best reasons for using such a dsp card!
You can also synthetize your guitar track with the flexor Pitch to CV in the modular, use vocoder....it's a real paradise for guitar IMO.