Newb with a 225 needs assistance
It's definitely not valve noise — no way, no how. It's bigger and uglier than that.
'67 or earlier engine won't fit in front of '68 or later automatic transmission (small/early crankshaft counterbore vs. large/late torque converter nose).
'65 or earlier automatic transmission won't work in '66-up car unless you make major changes ('65 steering column or floor shifter needed to operate twin-cable early transmission).
'66-'67 automatic trans will neatly mate '67-down engine to '68-up car. Or, special custom torque converter can be made for the '68-up transmission to allow the '67-down engine to fit.