I just rebuilt a 350 short block engine and have installed it in the car. The engine runs fine . The problem is the forward oil pan gasket leaks . We beleive it is because too thin a gasket was installed accidently . In any case , is it necessary to completely remove the engine to replace the oil pan gaskets , or is there another technique , such a jacking the engine up to enable removal of the oil pan ? HELP

Are you refering to the front rubber seal that is between the timing cover and the oil pan? Kind of a half moon shape? I'm assuming a standard pan gasket...not the one piece type.
If so, there are TWO different thicknesses of those seals. You just probably need the thicker one (newer model oil pan). Can be easily changed with the motor in the car.
Drain the oil pan, remove all pan bolts except the two rear corner ones...just loosen those. Drop the pan down slightly in the front and swap seals. Be sure to RTV the corners. Now reinstall all the pan bolts, tighten and refill the crankcase.
Simple job...just did one this week on a '69 SB with driver side dipstick.
Oh yeah, the driver is a dipstick, too! ;)