68 dart 360/904 vibration

hi all i live in the uk. last year i bought a 68 dart from san gabrielle pasadena ca. this car had a tricked up 360 with 904 trans and 3500 stall converter with sfi spec flex plate. the engine had spun a rod and when it arrived in the uk it had also froze up due to bad wether and cracked the block. sine then i sourced a 71 360 block and built a new motor using a eagle steel crank and eagle i beam rods je pistons comp cams 292 cam edelbrock performer rpm heads and intake etc etc
i have installed the engine and it has a vibratinon that clears at 3000 rpm this is a violent vibration from 1000 up to 3000
since running it it lost drive and broke the trans from the bellhousing and lost drive!!!
can any one shed some light on this situation please!