I once replaced a flex plate in a friends '70 Impala long ago,by putting 5" long 3/8" bolts in place of the two upper bellhousing bolts and removing the rest of them,then I put a floor jack under the transmission pan ,and pulled the trans back as far as it'd go till the slip yoke was right against the tail housing..the cooler lines had enough slack to leave them connected,someone had used rubber hose to splice them before evidently..
It sucked to get the bolts out and especially back in,getting them started was a mother--but was possible,using a long box end wrench,but it only took us about 2 hours,doing it outside on the side of the road,with the car parked with the two passenger side tires up on the curb...that was our "lift",that and a bumper jack and two pieces of cordwood for jack stands...the flex plate failed when we were driving along,and luckily we got it parked that way before it wouldn't move the car any more..