Not unheard of at all, especially if your car was expensive or one of the higher horsepower cars. People forget that Muscle cars were getting expensive to insure by 1970 and most mustangs sold were the plain jane variety.
One of my cars is a loaded 428CJ car, ordered as a Dealer Demonstrator on 9/09/96 and built on 9/29/69. Even if it was used as a dealer demonstrator till the 71's were available in june of 1970, it still took another 10 months to sell. It was finally sold on 4/16/71, 18 1/2 months after it was built.
My M code 70 Mach was a Basic/off standard order, made to keep the line running on a slow day then pawned off on a dealer to try and sell. It was built on 1/26/70 and it was finally sold on 12/02/70, 10 months later.
My newest acquisition, a grabber green loaded 70 CJ was a stock order for a local dealer. It was built on 9/11/69 and finally sold a year later on 9/16/70.
I think the problem with both CJ cars is that they were both loaded, and pretty expensive for a mustang in 70. Both have AC, AM-FM radios, and automatics, plus the 428CJ engine. Those 4 options alone total $1192.00, which added 36% to the cost of a base Mach1, which was a pretty expensive mustang to begin with. ( both cars have some other options so they were even more expensive..)
The 351 is kind of an odd optioned car, in an odd color combination (M code, AC, Auto, drum brake car that's medium gold with a ginger interior
) I bought that car off the original owner and he had the dealer add 15" wheels just to convince him to buy it. He had it painted Calipso Coral when it was 4 months old...
JMHO, John