According to Galen Goviers production option book (8th edition) there were 2507 convertibles made.
There is no specific break down for the V8 engines in those cars.
But the numbers for the 200 & Signets are lumped together.
The numbers are:
200 sedans (39,392), Signet (13,045), Signet convertibles (2507)
For v8s:
273-2bbl (7,633) with 7,038 auto, 298 3-sp manual & 298 4-sp
273-4bbl (446) 248 auto & 198 4- sp manual
I don't expect the ratio of 6 to 8 cylinders to be the same, I think the convertibles are much more likely to have a V8 just because they are a more expensive car to purchase but really nobody has it documented.