To slightly follow up on this:
New opener arrived last night. Played automobile shuffle so I could get the car out of the garage (not drivable again, mind you), laid all the parts out, assembled the rail system, wrapped the rail/motor/pulley with the chain, swapped the header mounts. Chain was looking about 4" too short. Scratched my head for an hour thinking what I could have possibly done wrong trying to set it up. Nothing. Old chain is different beefiness, and is a full chain - new one is a chain and cable combo - I can't just swap that over. So I went ahead and swapped the sensors and wall button.
Called Chamberlain this morning, and they said the measurement I got is a good 12" (foot) shorter than it should be. Awesome. I honestly don't think there's a foot worth of chain that I can adjust out, but I have a chain splitter somewhere so too long is better than too short at this point. I did not get an extension kit or anything. Pure off the shelf (but through the mail) one-box standard setup.
So now I have a car that has exposed ignition wires a hair away from the body and hood that I need to back in and out one more time, two partially assembled door opener rail systems attached to their motors laying on the garage floor (big deal since I have the smallest 2-car garage that you can call a 2-car, but have the car, a jetski on trailer, two spare Miata blocks, 2 jetski motors, 2 lawn mowers, and a ton of other crap in it - floor space is very limited right now), and still no electronically working garage door for the next 5-7 business days. So much for customer service.