massive rear main leak

[cfordman]

thanks for the latest help, as I am waiting for a cam gear to arrive I pulled the pan checking on what had been done before I bought the car. idling in the garage for 10 to 15 minutes produces an oil slick 8 inches in diameter. I found the original rear main cap tube in place and extremely loose( has a check ball in it) the threads are tapered badly and stripped. I don't know how it stayed in place but did not seal to the cap. I installed a new tube and it fits tight. could this alone cause the problem. there is no blow by at the filler and the motor does not smoke. there is a small amount of thrust on the crank. new pistons had been installed by the last owner.I have an aluminum rear seal from obsolete parts. can I drop the rear main cap with the motor in place and clear the bellhousing and if so is it possible to spin out the old seal and install a new seal? lastly is it possible to get an accurate valve gap reading from measuring between the low spot on the cam and the lifter bottom without having to pull the side cover?