I added a line in your .asc file that has .subckt <filename> etc. This makes it a subcircuit that can be used/called.
I added a line at the end that is .ends stating end of subcircuit.
I then saved the file as a .sub file.
Now you can put the symbol in the sym folder and the subcircuit in the sub folder. The symbol is available in components and it calls up the proper subcircuit and all works great.
Attached are the symbol and the subcircuit files I modified.
Thanks for the work on the getting the circuit modeled. They seem to work great.
W3JDR Bright Ideas 
I’m working out the details for a single band 30m WSPR Transceiver based loosely on the Small Wonder Labs PSK-20 I have. I used your SA612 model with KI6LIA’s changes. He was using LTSpice IV so I had to make a minor change to his .sub which called out a standard.bjt model that has moved since version IV.
I made a crude 612 LTSpice model that I posted on my website 10-12 years ago (KE6US.com). I based it on the Gilbert Cell schematic in EMRFD. It worked OK, but I didn’t have time to refine it. Solder Smoke talked about it so it wasn’t surprising to see they are still right on top of things by mentioning your far better model.
Very very interesting site. I’m up to my ears in projects right now, but I have to find time to fit your quad bus-switch mixer and the TPP amp into some bench time.
Eric
I just saw your comment. Thanks.
I’m building a radio right now that uses the Quad Bus Switch Mixer in the front end, driving a homebrew 40 mHz roofing filter, then down to 9 mHz with another buss switch Mixer, then demod using a KISS mixer also using bus Switches. Also see the BiDi amplifier using bus switch that I just posted. Works great! I have another in the oven using a MAR6-SM pellet amplifier.