Puppies are born 63 days post-ovulation, not 63 days from breeding. Your bitches may be further along (or not near as far along ; ) as you assume if you are picking a due date off of the breeding date. Ovulation can be identified by progesterone testing-- a blood draw performed by your vet that gets sent off for results the next morning. The eggs are ready for fertilization between 24 and 48 hours after ovluation, and sperm from a natural breeding can live up to 7 days. Without knowing when a bitch ovulated it is difficult to get a narrowed down due date because she could have been bred several days prior to ovulation and if the sperm are long lived the breeding will take or she could have been bred up to 4 days post ovulation and had the breeding take-- that is a span of 11 days!
If the bitch has been bred before another way to figure out ovulation date is to count 63 days back from when her previous litters were born and, assuming you kept notes on her heat cycle that time, you can figure out about what day of her cycle she ovulated on. Not so accurate as blood testing (particularly since it can be difficult to pinpoint day one of a cycle) but still gives a better time frame than dates of breeding.