This may seem obvious,but needs to be asked,was the carb readjusted after the exhaust work was done?
Any changes in the exhaust system could have easily affected the carb settings...
Go back to tuning 101,check and reset your timing,then re-adjust your carb,idle mixture/idle speed etc.,and see how it runs then,you may need to adjust the carb further with rods or jets or such if it still exhibits issues.
It's entirely possible you bumped your distributor position doing the plugs or something too.
The 670's use the (R)45S "gasketed" style plugs like David mentioned.
The T in the R45TS plug stands for tapered seat,like David mentioned that is a later model plug design ('72 and up).
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