Do yourself a favor. Assume that doing the job will loosen the base gasket. Buy what you need to pressure test a motor with the money youd save on blowing up your motor.
I did this job. I was careful not to disturb the base gasket. However, i 4-point seized a piston after. Its very possible that i already had a case leak on my 10 year old salt water motor before hand, and it was unnoticable stock and putting a pipe, head, etc on it all at once cranked enough extra power to make that tiny leak a problem. After the stuck piston, the motor was tested befor disassembly and it had leaks. I didnt test it and the person doing the work for me didnt say exactly. Regardless, there is no way to know if the leak started before or after.
What i learned however, is even if you are doing a non-girlded head, its a good idea to leak test older motors before adding a bunch of high power parts. For the money spent on 2 pistons, a bore and gaskets, i could have bought the stuff to test it with plenty of money left over. And, even if you do have to replace the base gasket, you should leak test ater a rebuild anyway.
