Here's my best guess. 5555cc is 339 cubic inches. That displacement in a Chevy smallblock motor was used by several sports car racers in the 50s and 60s, among them, Lance Reventlow's Scarab, the Echidna Special, Lister-Costin specials and a couple of Devin-bodied cars. What I've read says that the venerable 283 motor was bored and stroked to achieve that number. In order to get it, the bore was increased from 3.875 to 4 inches (that's an awful big overbore for a 283) and the crankshaft stroked from 3.00 to 3.375, an increase of 3/8". This gives a total displacement of 5558 cc. Some pretty serious bore/stroke increases for that time.
The Scarab motors were built in 1957, before the 327 was available (1962.) After '62, it would have been less of a chore to use a 1/8" stroker crank in a 327 to get that displacement size.
I've built a lot of smallblock motors, but I've always used a 350 or (my favorite) a 400 as the basis. I run a .030 overbored 400 (406 cid) in my race car. I don't think I'd want to stretch a 283 out to 339, but apparently it was done and John Staver won the 1958 SCCA B-Modified championship in the Echidna special with one of those hand grenades.
I ran across one other 5555cc motor; an Infiniti V8, but it's a much newer design and I'd be very surprised if that was the basis of the motor in the Morris. There are probably other motors that could be modified to get that number, but I didn't run across any in my search.
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