Taking a heat gun and warming up the end of the arm that attaches the motor to the mount might help things along, too. Don't get it too hot, obviously, but if that bolt has been in there for fifteen+ years, there may be corrosion, or possibly some red loctite. Either way, the aluminum will expand more rapidly than the steel bolt, so a little bit of heat, here, could go a long way.

If all you have is a little L-bend 8mm (or is it ten?) allen wrench, then feed the long end up through the subframe and into the socket-head fixing bolt. Then take a deep-well 3/8" drive socket (8 or 10mm, whichever is the same size as the allen) and put this socket on the short end of the allen wrench. Use as many 3/8" extensions as you have on hand to lengthen your "handle" (insert the extensions into the socket on the allen wrench).

When jacking the motor, use a pretty big block of wood. I've got a piece of 2x10 that is perfect. I like the piece of wood to be bigger than the pan.

I just did this on my wagon a couple months ago, so I know how you feel. If you start to get too frustrated with it, just walk away for a day or so. Your brain will keep working on the problem in the interim; you'd be surprised what kinds of insight may materialize....

Jay.