Check the injector (inside the throttle body)
no.
have the cylinders bored .030" oversize and fit it with new pistons.
Yes, that should be ok.
If the pistons are hitting the crankshaft, either you have1. the wrong pistons (350 pistons are relatively short butit is possible to purchase pistons with the same bore but a longer skirt... in which case they would hit the crank)or2. the wrong crankshaft with too much "throw"
yes when the engine is out and upside down
Depending on what the engine was used for, most likely dish top pistons. But flat tops ARE possible.
No, 350 has a substantially larger bore.
No. Standard 350 engines have 4.00" pistons, standard 400 pistons are 4.125".
no thay are not.
No the diameter of the crank journals are different.
Hopefully someone with the correct answer will improve this answer, but a shortblock weighs 290. That is a 350 block with crank, rods, pistons, and cam.
You buy the correct crankshaft, rods, and pistons and install them in your block.