Sizing for a harmonic balancer, 283 Parts #3

Good Day All
This is part 3 of the project of getting parts for a 283 long block, some from a tired '76 350 in the '64 Impala.
My mechanic is very busy so in order to get my car fixed and out for other cars he would like me to get some of the parts that take longer to order like a harmonic balancer.
well when I called the parts place they had 4 for a '64 283 ranging from a 6.25, 6.125 ring unriveted . The engine long block is still at the sellers does not have one. When told my mechanic today that about the options he told me since he would be using the same pulleys from my 350 to crawl underneath '64 find and measure and then order. does that sound doable? I used engine cleaner to get some of the gunk off the engine before the swap -out but I don't know if I can find it. I know now what a hb looks like but is it possible to get an accurate reading with such fine differences. I could probably order all 4 hbs then return the ones that don't fit but then I would get hit with re-stocking fees..
Thanx, rick

You should be able to measure it accurately with a good tape measure or steel scale. Is there any chance of borrowing a puller and taking it off? The whole process takes like 10 minutes and is not difficult.

Bob