It can be done in the car, but it is so much easier to remove the engine from the car. Cleaner, easier access to everything. Remove it. It is the easiest way to go about doing a pot swap. Get yourself a copper base gasket .020" and some anerobic sealer avaible at any parts store.
http://www.permatex.com/products/Aut...et_Maker_b.htm
Resist the urge to use any type of silicone.