360 engine vibration at ~3000rpm

360s are externally balanced engines. To mate up your 360 to your 727, you either need a Summit Racing cut out solid flex plate for the 727 (not 904) to offset the crank external balance or you need the weighted torque converter to go behind 360s.

If you have a factory 4 eared flex plate then you need the weighted torque converter. If you have the Summit Racing cut out solid flex plate for the 727 then you need the neutral balance (no weights) torque converter to go with that.