The end thrust play is caused by one of 2 things. Either a worn thrust bearing or worn thrust surface on the crank.
I assume when you say slop between pistons you mean between the rods. Again 2 possibilites. First is the sides of the rods are worn or second the crank is worn out.
First thing I'd check is the rod and main bearing clearances. You can get a strip of plasti-gauge at the local auto supply and pull each cap one at a time and clean all the oil off every part of the bearings and caps and rods where the bearing rides and cap goes on then lay a 1/2" long piece of plasti-gauge on the crank surface and torque the cap back down to factory specs. Then remove the cap and use the guide on the plasti-gauge package to determine your clearances. For the mains use a 3.4" long piece of the plasti-gauge. Normally rods should be between .0015" and .002" and mains between .0015" and .0025". More than that and your getting pretty loose but I've seen racers run up to .0025" on the rods and .003" on the mains without any knocking but those engines get tore down every year to rebuild them. You don't want that much clearance on a sb street engine. If all the bearings check out ok you need to inspect the pistons. Maybe a cracked piston. Sometimes if it's a cracked piston it'll leave scratches on the cyl wall you can see without taking the piston out.