O.p., did you look to see (or ask the owner) what rear it had?
Real easy (REAL easy) to tell a 7 1/4 from an 8 3/4. Smooth round backside, 8 3/4, cover on the back with bolts, 7 1/4.
Unless you are planning an engine swap, a 7 1/4 could be fine. Of course an 8 3/4 is preferred.