Chevy 400 rear main leak or oil pan?

Hi everyone I'm new to this site I came over from the Pontiac forum because I got tired of people telling me how I should of installed a Pontiac 400 in my 79 tenth ann. trans am instead of the chevy 400/425HP that I have now.
Can anyone help me.... I just purchased a chevy 400 30 over from a place called Indymustang.com well I installed the engine and it leaks from the rear of the engine from either the oil pan or rear main. I thought it was the oil pan but I replaced the chrome pan to stock and used the one piece fel pro gasket-still leaks. The engine only leaks when started and not when sitting. I'm so frustrated I've had to work on this pig every day and havent been able to enjoy it. I don't really want to pull the engine. Can the rear main seal be bad on a new rebuild? Can you change the rear main seal with the engine in the car? Any help would be appreciated.:confused:

Had the same problem with the 400 I had in a 62 pickup. Took me 3 rear main seals to get it fixed. I had a lot of clearance under the truck, wouldn't want to guess about about a Trans Am

What type of rear seals did you finally use to fix it? Also did it appear that the oil pan was leaking? It sure looks like it is after looking under there with the engine running. It is hard to tell. But you were able to replace them with the engine in the truck? If so did you just jack up the engine and remove the pan? very frustrated ready to junk the pig!

I used Fel-Pro seals and the only reason it took me 3 was because I screwed 1 up and a 283 flywheel caused the first one to go. The pan was stock and I used red rtv instead of the stock gaskets. It didn't leak and I had the fornt end in the weeds, but had plenty of room after it was jacked up.

Cool !! I think I'll try the Maxi pad, but does the crank have to come out to get to the main seal?

Rope seal or rubber seal?

No you don't , and Oh welcome onemengang. There is a tool that looks like a wire chinese handcuff with a T handle. U have to pull the oil pan of course, then you pull the rear main cap, pull out the old seal. Feed the tool through and slide the new seal in the tool. As you pull the tool tightens and pulls the seal through. They don't cost much.:D

thanks for the replies fellas... Henry it is what I believe a rubber seal it's blue.....Brickman That sounds to good to be true, you have done this? Could you get me some more info on the tool and what type of seal to use. How much do you think a mechanic would charge to do a rear seal if I pulled the enine and brought it to them?