I like it, but i still think you should stick with the Perkins, as the Perkins is a good motor and will last very good...

Is it impossible to fit a 6BT in it ?

Nothing is impossible! LOL. The Perkins 6.354 is pretty much the exact size as a 5.9L 6BT Cummins, though the Perkins is roughly 200 lbs heavier due to the it's beefier block. Both would almost certainly require cutting the firewall of the cab for clearance.

As good as the 6.354 Perkins is, without a harmonic damper (which I do not have and are impossible to find), it likely wouldn't survive 100 miles of street driving before the crank broke.

Sorry to hear that, but I wouldn't give up on the Perkins yet. Especially considering what you got into it so far.
If it's neutral balanced, couldn't you take the crank, flywheel and adapted damper to a machine shop to have balanced?

Thats an awesome project I think perkins are awesome engines I have a MF 180 with the 4 cylinder diesel fun little tractor cool project!