360 vibration issue

Hello all. i need some tips about my 72 scamp vibration. the car vibrates real bad between 1500 qnd 3000 rpm in park. 55 to 65 mph is pretty rough on the vehicle. the motor is pretty stock internally. 360 eng, in front of a torqueflight 904 the 360 swap from the 318 was done maybe 10 - 12 years agoi remember using the 318 balancer on the motor instead of the proper externally balanced dampener. replaced that just this year and the vibration wasnt so bad but it didnt solve the issue. i have brand new stock motor mounts and a high performance trans mount. it seems that the vibration became very aggressive once the performance trans mount was installed. the flexplate is a high performance B&M plate which is like a drumkit symbol with a small round pie cutout. didnt use weights on the convertor with application because of that. hope this helps. thanks for any tips