It sounds like your friend used the regular bottle of seafoam. These are for adding to oil and fuel. You need to use the spray version for intakes. Also, you need to hold the idle high (1,500 rpms or so on yours).
Vacuum leaks were the first thing I thought and then read on and found that to be a big issue for you. Replace with silicone ones for longer lifespan.
Fuel filter should be replaced if you don't know when it was last done regardless of the fuel treatment use.
The shudder you still get when running might be a miss, base timing could be off, carb tuned poorly, or just worn motor mounts. Hook up a timing light to each plug wire and pull the trigger. If it's flashing steady you don't have a miss. If the flashing studders you have a miss on whatever cylinder you're on. Check cap and rotor on distributor. Use the timing light to check your base timing and make sure it's within spec. If your motor mounts are old, tired, soft, broken down, they need to be replaced. Carb tuning is a bit of an art. Learn how to do it. Pulling and checking plugs can help tell if you if you're lean or rich.
