Hi Ken:
I personally wouldn't spade and transplant 8-10" diameter, multi-stem yellow-poplar trees that are 20+ feet tall...most equipment can't capture enough root ball to support all that above-ground biomass. I'm sure there are many people out there in the tree care arena who will disagree, but I believe you're wasting your time and money considering how fast yellow-poplar trees grow...just my $0.02