My '73 142 sports front and rear seats from a 242GT, so I've BTDT.

Rear:

The seat bottom is a direct fit.
The seat back requires that you take two small brackets from the donor car. Each mounts with 2 screws, but I wound up using an existing hole for one screw in each, and drilling only one new hole per.

Note that I did use the rear seat from a 242. No idea if a 244 rear is the same.

Front:

The 242 seats went in with a combination of parts. You will need:

Pair of 242 seats. 244/5 seats CAN be used, but they don't flip forward, rendering your rear seat useless!

2 sets of 200-series PASSENGER SIDE adjuster tracks. Can't use one from the driver side. The lever-style height adjustment makes it impossible to use. The height will be adjustable, just not "on the fly".

2 sets floor brackets from a '74 140 or '74-5 164. This will NOT work with your '73 (or earlier) brackets.

How to:

Passenger side:

Take the floor mounts off the 242 seat, leaving the sliders in place. Loosen the height adjustment bolts, but do not remove. This allows some play to line things up. Attach the 140/160 floor mount to the slider tracks. Snug everything up. Install seat.

Driver side:

Remove bracket and sliders from 242 seat. Install sliders (minus floor mounts) from 200-series passenger seat. Proceed as with the passenger seat above.

YES, it's a pain in the butt, but as you noted, 142 seats are hard to find. Then again, 242 seats aren't easy either. I bought the whole 242GT (parts car) just to get the seats! But your butt pain will be assuaged when you plant your butt in some nice 242 seats.

More questions? Just ask.