You need a methodical approach to the the problems here; it sounds like
there are a lot of possibilities in your mind and you're sort of going from
one to the other. Start with the timing, and pretend the thermostat isn't
even there--just set the timing to about 5* BTDC, and make sure it's right.
This alone could solve your problem. Once you KNOW the timing is set, look
at other things like vacuum leaks, etc., one at a time, not all together.
BTW, the thermostat is not the direct cause of your problem, though a
thermostat stuck open will cause running problems once the van tries to go
into closed loop mode. If your low-power problem is happening even when you
restart the van, it isn't your thermostat. Again, a methodical approach is
key, otherwise you'll drive yourself nuts, as most of us have.