As Truckernix posted, the torque spec for 235 changed in 1953 from 75-80 to 90-95 FT LBS. Be sure of your block year. However it should have nothing to do with the bolt breaking. If these are 1/2 Bolts, 90-95 is beyond the range of a Gr 5, even a fine. Maybe they are 9/16. The torque charts don't show that high but there are a lot of variables. Stay at 90.
You should have been able to use all the old bolts. However, especially with head bolts, PO's have "stretched"/overtorqued them, and when you re-torque it breaks.
This is not a disaster, as you can see, lots have done it. Be careful when a bolt gets a rubbery feeling from now on or something doesn't seem right. No jerking when tightening.
