I removed subframe with engine and gear box.
Now I have to take off TH350 gear box from 351 engine.
What should I expect here?
I mean do I need to support some parts or is it just OK to remove the bolt and I will take off the gear box. Is it heavy?

 

Unbolt flex plate from torque convertor, then unbolt trans from engine block.
Yes, trans is heavy, approximately 300 lbs.
Set assembly shown on floor and separate. BE CAREFUL.

 

This and it's a 350 not a 351,351 is a Ford small block.

 

You're correct but it's only 1 Cubic Inch LOL

 

2 more questions on this TH350
1- The mounting holes for the torque convertor on the flywheel are oblong but quite consistently and without sign of being deformed. 1 seems to be round.
2. Flywheel to block distance, as measured through the mounting holes to the block is different left to right.
I have a difference of about 1.5mm/.060"
If I turn the flywheel/engine 180°, the difference remains but moves 180°. I assume the flywheel is kind of bent. Is there any concern to have here?

 

If me, I'd replace the flexplate.

 

Checked pricing for flywheels
Priced so I can replace it and loose doubts.
But do they need to be balanced? Or is it just a bolt off/on affair?

 

Bolt off & on. But getting it balanced is not a bad idea.