First of all I don't know the 'right' answer for sure, but I can share with you how I did "this sort of thing". I sort of faced such situation. Actually I was 'converting' a piece of telphone line test unit into something useful, heh heh
, I also wanted to try to built a guitar overdrive ala ibanez TS-808, and then I've got a few extra ideas on the way.
The logic here would be (as I understand) - add attenuator (pad) on input ("convert" line level down to instrument level), and then ad a "guitar preamp" at output. So you have instrument and line inputs option and also you have instrument level and line level outputs.
Here's the entire schematics of the unit I've built:
http://www.mzentertainment.com/pics/silicon_driver_overdrive_distortion_spring_reverb/dr_zee_silicon_driver_guitar_preamplifier_overdrive_spring_reverb_schematics.gif
here's the control panel of the unit:
http://www.mzentertainment.com/pics/silicon_driver_overdrive_distortion_spring_reverb/dr_zee_silicon_driver_guitar_preamplifier_overdrive_spring_reverb_control_diagram.gif
Now, when you look at the schematics, ignore all the things that related to relays, VU-meter, the entire spring reverb section, headphone amp. So if you just look at Drive Section and Tone Control Section - what you see there is pretty mach ibanez TS808 overdrive, but with few mods and rather freaky drive adjust and tone control ..heh heh, but in general you can view it as TS-808 pedal: everything between first and secong sc1815 transistors (minus relays and freaky "controls"
).
So as you see ther, I have line level input, which is two resistor pad (also I have 0.1 uF cap there instead of 0.022 for instrument input), and then after second sc1815 transistor (that's where TS-808 "ends"
) , - next you see "Output amplifier" section . I have ther one transistor, followed by "no-gain" OpAmp (it also acts as summing amp for reverb 'return' there) , and then I have Line output and two resistor pad (attenuator) for instrument output.
Again, I can't tell you exact design that may work for you, but only as general idea.
There are bunch of big and small issues all around, I must tell. The project may be simple but may be not ... Good luck if you do it.
Maybe you can find some simple guitar preamp kit and then put it all together with your delay kit into one case, plus some power supply, of course.
here's photo of
'silicon driver' :