it sucks.
i would start by breaking the bolts free on the mounts. and you have an option of taking the attachment cradles off of the motor...some say that makes it easier. i wouldn't do it. just loosen the bolts on the mounts themselves. then get a hoist and lift the motor up enough to get the mounts out. then bolt the new mounts to the cradles...then lower the motor back down onto the chassis (passing the mount studs through the holes in the rails). then bolt them back on.
this will probably take all day <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt=':)'>
Eric