No. The 400 is externally balanced, the 350 is internally balanced.
Take the crank to a machine shop and have the main journals turned to accept 350 bearings. Might be a good idea to have it balanced while it's there.
No. The 454 is an externally balanced engine, the 350 is internally balanced.
remove the enigine and then take out flywheel bolts
No
NO
NO.
yes
yes it will. chevy V8 engines all shared the same bell housing bolt pattern. you may need to replace the flywheel though as 454 were usually externally balanced where 350 was internally balanced.
yes
No it's not.
No.