can't pull pistons from below cos of this lip edge that forms the main bearing as skinner mentioned, pistons can only be removed from above with head off
to swap the crank with the block still mounted to the chassis will be very very difficult.
cos the crankshaft goes through the lower timing cover and the rear crank seal, you have to remove both these ends before ur able to drop the crank.
removing the lower timing cover requires detaching the RH engine mount, crank pulley, timing cover and then reattachingthe mount.
removing the rear main seal requires removing the gearbox & fly.
then you'd have to remove the head cos the pistons only come out from above.
then with the gearbox removed (which supported the engines LH) you'd have to find a way of supporting the LH of the engine block and stop it from rolling back forth.
then undo the crankcage bolts and carefully lower the crankcage and weighty crankshaft.
THEN go through basically a full rebuild sequence to fit the new crank and everything else.
imho I'd suggest either lowering the whole engine/gearbox assembly off the car via several jacks or hire/borrow a crane briefly for few quid, rebuild the engine away from the car and reinstall with the crane or jacks.