Brett,
Are you certain it's the seal? Many oil pan leaks are blamed on main seals...
Was the block align-bored or align-honed? If so, was the "special" seal for the 400 that's BEEN align-bored used?
Yes, the rear seal can be replaced in the car. Get a service manual and it will describe the proceedure in detail. Chevrolet manual would be the best one.
Scott,
Bad idea to use brake fluid inside an engine. While it's true, it will "swell" the seal, it will also swell the o-rings and valve stem seals. It also will eventually evaporate, causing the parts to shrink more than they swelled. Best thing to do is just fix it...
Jim