S13 Misfiring and Overcooling - Please Help

[m3gav01t]

I have a '93 240sx and I just had a S14 KA24DE engine swapped in by a shop. They kept the S13 wiring harness, distributor, and manifold on. The engine was in an auto car, mine is a 5 speed. Before swapping the engine in, they did a compression test on it and they said everything looked good. Here is a list of problems I've had since I got the car back 2 days ago:

1. The engine was misfiring and delivering very little power to the wheels. It was stuttering and bogging, to the point that the car would buck hard while trying to accelerate. It was not doing this when the engine was in the S14. When I had the original engine in my car, it did misfire slightly, but nothing major and I had decent power. The shop replaced the spark plugs and spent a lot of time calibrating the timing on the engine in order to try to fix this. I replaced the spark plug wires, distributor cap, and rotor, which seems to have fixed the problem while driving as long as it's not super cold out. However, the car still misfires at idle even in warmer temps.

When I drive to work in the morning, though, it's like 20 degrees Fahrenheit, and the engine still misfires and has very little power. It almost stalled out both yesterday and this morning when I started reversing out of my parking space. If I keep the rpms above 3k, it seems to misfire less and i have more power, but it still doesn't feel like it's pulling quite right. After driving for about 15 minutes, the problem becomes less noticeable, I think because the engine warms up slightly, which brings me to the next issue.

2. The car is overcooling. If I let the car idle, it will reach operating temperature. However, as soon as I start driving, the temperature gauge drops back down to below C, like where it is when the car is off or cold. I also have no heat. The shop replaced the thermostat with an OEM one, so I don't think it's stuck open. I drove for an hour at like 50 mph and the temp gauge never budged from below C. The outside temp was in the 20s and my old engine would reach operating temp even in like 0 degree weather. If I shut off the car after driving for a while, let it sit a few minutes, and then turn it back on again, the temp gauge will be at the first or second tick mark above C or so. This, of course, doesn't last as soon as I start driving again.

3. When I take the rad cap off and start the car from cold, within a minute or less coolant begins overflowing from the radiator. This also happened before the engine swap, but I thought it was due to the fact that the block of my old engine ended up having a hairline fracture, which is why i did the swap in the first place.

Here's what's been replaced either right before or during the engine swap:
-Thermostat
-Thermostat housing
-Water pump
-Clutch fan
-Upper and lower rad hoses
-Spark plugs
-Spark plug wires
-Distributor Cap and Rotor

So far, the only thing I've checked is that the clutch fan isn't seized. Before the engine swap, I checked that the radiator wasn't clogged.

To start, I'm going to try to re-bleed the cooling system myself tonight and also check if coolant is flowing through the rad from a cold start. From there, I have no idea what to do next, so any help is much appreciated. Thanks in advance.