It might be the spider gears, sorry to say it but Ive had similar problems, sounds just like a u-joint. Pull the cover make sure the pin is still...
Check the fuel pump, are you getting good fuel flow at 6-8psi?
Have a SBC block originally 350.High nickel casting 4 bolt main. Factory size (4" bore). Needs macine work(boring)but has a little cross hatch...
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