Muffler body length

It is muffler time and I have already decided on the type, shape, pipe size.
I just need to find the maximum length that will fit under my 1970 convertible.
Replacement mufflers have a 14 inch body length—not the total length that includes the pipe nipples---- but just the length of the body of the muffler.

It seems that the next size in length is the 18 inch body length.
I could not find anything in between 14 and 18 inches.
Will an 18 inch muffler with 2.5 inch in/out and a 4x9 oval body fit in the muffler recesses?
It has been a long time (decades) since I seen an original muffler on a cougar but I thought that I had seen OEM mufflers that were a bit longer than 14 inches.

I searched the site and I found a lot of different systems but I could not find a definite answer on muffler body lengths.
I did not look up all of the part numbers that were given but the ones I did look up seem to use the 14 inch length.

I thought that I would ask before ordering mufflers that would end up hanging on my garage wall instead of under my car.

Cheers all.
Mick

You may want to consider measuring the location you want your mufflers installed in and use that to determine maximum size.