Sounds like the pistons outgrew the cylinders. The first start on a fresh engine is always hard- very hard. You let the engine idle and circulate oil and do not rev the engine while it runs in, about- I'd say a half hour. You did use assembly lube on the bearings as you put the motor together didn't you? Loosen and re-torque the con-rod bearings to make certain none of them are too tight. Let the engine run at idle until it will run smoothly, before you try to drive the car. Wait til tomorrow when the engine is stone-cold.