I've done it both ways, and to me it is easier to install them separately. Especially in a B body or E body. I drop the motor in first, then put the trans in from the bottom on a floor jack with a piece of plywood just big enough to support the transmission pan. Get it on the dowels, then put the lower bolts in and tighten them. Then jack it up until the tailshaft just touches the top of the tunnel. Then go up and stand on a stool and install those 2 upper bolts. Then let it back down and secure the transmission mount and crossmember. Jacking it up as high as you can after you get those bottom bolts in will give you a little more working room between the firewall and the bellhousing. A short 9/16 socket on an air ratchet helps ease the cussing....lol.