Trying to establish a null modem connection between 2 C64s using 1 Commodore 1660 300 Baud modem and 1 Commodore 1670 1200 Baud modem.
Does anybody have detailed instructions on how to configure the modems for text exchange and file exchange?
"Null modem cable" may not quite be a misnomer but is potentially misleading: it is a cable that connects two computers via their serial ports without modems. It's not a null cable for modems, it's the null example of one of those cables you use to connect a computer to a modem.
So if you want to make it look like there's a phone network between two modems that are really directly wired, you're not asking about null modem cables.
If you just want to network two C64s then the easiest thing is directly to connect them via their serial ports. That's not technically the same thing as the normal usage of the term "null modem cable" because that term is usually associated with full-duplex RS-232-style connections, whereas this will be half duplex but half duplex is no barrier whatsoever to networking. Ethernet is also half duplex.
Although the software is customised, the cable you'll need isn't. Any standard Commodore serial cable will do — impedance is sufficient that the author had no issues even with five C64s and a total cable length of 25 metres. So if you have a 1541 then just use the cable from that.
I know this is anecdotal, but here is my personal testimony:
I built many circuits interfacing with the User Port in my teens (mostly digital control circuits using the data lines), and I'm pretty sure I interfaced my 128 and 64 together by directly wiring the two user port's TX/RX TTL lines together in a crossover fashion(along with ground, of course), but it has been a long time, so I may be remembering incorrectly.
I do, however, specifically remember connecting two 300 baud modems to each other on many occasions, using a simple RJ-11 jumper cable. I would simply send the off-hook AT command to each modem (and I think another set of commands to initiate handshaking), and the computers were able to communicate...