[Christ]

Get a NOS plate for the Civic's TB opening, or just drill/make one. You'll have to meter the flow, though, so a flow-rate indicator and AFR gauge w/ a wideband will help, there.

The other option is just to install an injector in the plenum, just after the TB that will flow the propane into the intake tract. IIRC, it's best to have it as close to the throttle plate as you can get it, so it has maximum turbulence to evenly distribute.

Remember that for best results, you should use the vaporizer from the tow motor as well. (Thing that looks like a regulator, and coolant runs into it.)