I couldn't get it out of the heat sink, so I bought this to poach it's
heat sink (since I'm having a lot of trouble finding replacement parts
for the MP SM V2): https://www.amazon.com/dp/B074NX68MP/ref=cm_sw_r_cp_apa_DDYkBbKYY5ZGS

reasonable

It worked well, but I noticed the throat between the heat sink and heat
block was shorter on the one that came (and the old one was welded into
the heat sink) meaning that the nozzle was above the stock shroud. So I
unscrewed the nozzle a bit so it hung lower.

oh no...

and then filament started seeping out

yeah...

​

what can I do to get this thing up and running?

Two paths:

path one, you finish the installation of this new heat block.

Double check the wiring for the thermistor is correct and good contact is made. jiggle the wires around and look for temperature swings. Also, the reviews of the package you listed talk about the screw that holds the thermistor can break the insulation and short it. take it out completely and see if it is still reading 350. If not, it was the screw and the best course of action is to use the old block's thermistor.

​

Path two, use the old block.

There's nothing wrong with the old one, you just need to clean the plastic off. reconnect everything and heat it to normal PLA temperature. use pliers and snips to take the larger pieces of plastic off the block. use cotton swabs and paper towels to get the thin, now liquid small amounts of plastic off. It's a deceptively low effort fix.

for the nozzle being too high, you just need a longer heat break.