Yeah.... You can do it in the car if it's a neoprene seal, but in the long run it's better to pull the motor and do it on a stand.
The hassles of doing it upside down laying on your back in the semi-dark with the blood running out of your arms causing pins and needles and then having the bloody thing still leak is not worth it. (yep, I've been there....)

If you pull the motor out you can freshen up a lot of other stuff as well like timing cover seal and gasket, maybe fit a new double row timing chain set or gear drive (or maybe a new cam and lifters!), plus you can give the motor a good clean and new paint job, paint the gearbox, crossmember, etc, etc...