360 balance problems

I have a problem with my 360, I don't think its balanced correctly.When I first got it running I noticed a vibration around 2800 rpm, felt like a wheel out of balance,not it, so I checked the driveshaft, not it, noticed the output shaft was a little sloppy,not sure what the specs are for that,so bought a , tci super street fighter tranny, tci 3000 stall, new driveshaft, and a new rear yoke, about thirty five miles later I blew the front pump seal out while warming it up in the garage to check the timing,while doing so I noticed the vibration around 2500 rpm and up.I'm thinking the guy who built the motor diddnt balance the flexplate correctly. my question is how do I get it balanced right? do I need to take it all apart again or is there another way.
I've had nothing but issues with this thing since i did the engine swap.
Its a mild race built engine, supossably 400 to 450 horse. any help would be greatly appreciated