Could me engine timing be inaccurate

[burnin oil]

Set timing as said then double check advance at 3500-4000 rpm. This guarantees that the distributer springs let all that timing in. SbC usually like 29-36 degrees total timing not including vac advance. I set to 36 degrees at high rpm then test drive and back it down from there. Many aftermarket distributors will have base timing at idle around 16 or 17 degrees. Honestly you don't need a tach. Set over all timing at a high rpm then note what it is at idle for future reference
This is the only way to do it unless you are recurving the distributer. The curve is dependent on the distributer wieghts and springs and on some distributors, the bushings