360 engine vibration

Hi, new to the Dodge dart and have an issue with engine vibration over about 1500 rpm. Now a little background first , its a 360 engine mated to a 727 transmission , im based in the uk and bought the car last year from Texas. Now it wasn't an original 360 ( I wish) or even a v8, car was originally a slant 6 904 box and 7 1/2 rear axle. someone has spent a bit of money uprating the back axle to 8 3/4 fitting a 727 trans and rebuilt 360 engine, most of it looks new is recently rebuilt, i think it was a stalled restoration, i got it from a texas classic car dealer.

now back to the vibration, the car is not not UK legal, so i cant take it for a run. I've read loads of threads on here re harmonic balancers and internal and external balanced cranks. ive tried the obvious and removed drive belts, checked firing order, plugs firing, exhausts not rubbing on body etc. I now started to look around the crank balance, the front pulley looks to have an external balance and ive rotated and looked all around the converter and cant see an balance weights, so i'm thinking I will need the B&M balanced flex plate, as it look to be a stock flex plate. any ideas or suggestions?