As I see it, there's more than a bit of "putting the cart before the horse" going on here. So, lets cool our jets and take a look at the facts, and the speculation.
the goal is to drive some kind of 180gr bullet over 1,400fps. Right??
Will be using a 6in Ruger GP100 for testing.
so you want 1,400fps+ from your 6" Ruger, using Lil Gun (because you have Lil gun).
The guy in the UTube video got over 1,400fps shooting a cast bullet and Lil Gun from his 4" Ruger. The load he used was over Hodgon's max load for a different (and jacketed) 180gr bullet.
You haven't loaded and shot Lil Gun and 180gr bullets from your 6" pistol YET, right?
So, you do not know if you will, or will not get the desired velocity from YOUR gun, YET.
I'd recommend starting the usual way, using the published data you have, and see what you get for velocity from your gun. You might hit your magic number somewhere within the existing published data. You might not. Only shooting those loads from your gun will tell you. Nothing anyone else got from their gun is relevant, other than as information of what something else (that you don't have) did.
to be clear, I've never used Lil Gun, so I won't say anything about it, one way or the other. And, I don't shoot 180gr bullets in .357 Magnum. I shoot 158/160 gr and 2400 powder for my heavy loads. So, while I don't have anything directly applicable to Lil Gun and 180s, I do have experience with the principles that appy.
And, at the risk of further beating a dead horse, when you are looking for a certain speed with a certain bullet and load one of, if not the, most important factors is YOUR GUN. Again, what the books says, with the data says is done with their gun, and that is different than your gun, Maybe only a little, but maybe a lot.
As an example of this, here's something I personally did. .357 Mag, 125jhp, 2400 powder, a top end load straight from the Speer manual of the day...
Speer tested the load with a 6" Ruger and their max velocity was 1555fps.
I tested that load from a 6" S&W M19. Velocity was 1620fps. (and the load was unsuitable for THAT gun, cases stuck badly)
I tested that load with a 6" S&W Model 28 Highway Patrolman. Velocity was 1670fps!! Cases did not stick.
I also tested that load in a 6' Desert Eagle. Velocity was 1720fps and function was flawless.
Point here is that different guns will give different speeds using the same ammo. What YOUR gun does, is an unknown, until you test it. Might not be as fast as the book data. Might be faster! If it's exactly the same, that is serendipity.
(nice but you can't count on it)
you seem to have a good grasp of how to go about working up your loads and testing. Do some test shooting and see what you get. We're curious about your results.
Good Luck, be safe, have fun!!