Check the valve lash. All pre-69 273s were solid lifter engines. The only hydro was the 69s
But if it ran fine before the mods then;
1) confirm the firing order
3) confirm the reluctor gap
5) confirm the timing ring on the balancer is tight
If all is good then
with the timing light hooked up,and the Vcan defeated, slowly bring the Rs up. A bit of jumping around is ok like 2 to 4 degrees. But if the timing is jumping 10, 12, of out of sight and the strobing shows dropped sparks, AND, AND the engine won't take rpm very well and starts bucking and farting,etc, then:
Then back to; If all is good