First time mods on my 318 and it won't start...

Hi folks

Backstory:
Driving our cars can't be enough, right? So we have to start tinkering around, right? Well, I do. Perfectly aware of the fact, that I often only have a rough idea of what I'm doing (having learned most of what I know out of books, forums, youtube vids, ect. - things will go wrong, but I want to learn and have to start somewhere...), I've formed a plan to replace the 2bbl setup on my 71 Dart to a beefy (ahem) 4bbl. Goal: a wee bit more power and overall good performance; no race car. I intend to drive it on the street only. Oh and fix some of the oil leaks around the block; the poor thing is a self-moving oil spillage...

Now then, after reading through a gazillion forum posts about what parts I should get, the following things had been ordered:
The ignition already had the Chrysler electronic ignition, stock distributor and stock coil. The fuelpump is still original, headers are stock but exhaling into dual exhausts.

Ripping it all apart and cleaning up was surprisingly easy (remember, I'm doing this for the first time, no help from any friends or other physically present advisors, only "The Internet"), the manifold went in there, carb on top (linkage is still a mess...), as far as I can tell everything is connected and ready to go for a first engine test.

I've bought a used carburetor as it was an okay deal and I thought I could save some money while risking "issues" carb-vise. The previous owner disabled the choke linkage; I couldn't figure out how to reconnect everything for it to work so I just left it as it was for the time being. I didn't take the carb apart (they still seem overly complicated to me at the moment). It looked clean and not much used so I had high hopes that it would run out of the box. I am quite sure it wouldn't run nice and smooth but it would run...

The issue:
I turned the key, the motor started cranking. After a few turns I checked the carb; while opening the throttle lever gas squished out - seems to be working. Now I wanted to properly start the engine and cranked it again. It immediately fired up but died as quickly again. I did this a few more times, always with the same result. It seems to come alive but dies immediately. Apart from the vacuum advance hose (which has to be replaced as it is too short and brittle anyway), valve cover breather and the transmission linkage I think everything is hooked up.

I've filmed one attempt, maybe I can make it available somewhere.

Before I start rummaging through the engine bay without really knowing what to look for, does what I'm describing here sound familiar to anybody so you could point me in the right direction to check?

Sorry for the long read, just wanted to throw in all information I thought to be relevant...

Thank you!
Timo