What goodies do I need to build a Fast slant 6

300RWHP, is more than 350 crank.
350/225=1.555 hp per cube.
And OP wants it to be a streeter........
That would be like a 318 at 495hp, or 360 at 560hp,
for which good heads are already available.
IMO, a NA 225 at 300RWHP, for the street; is not feasible. And even if you built it, it ain't gonna be a DD. Probably not even a good weekend bomber. And for me the big deal is waiting for the rods to come flying out the side of the motor at the rpm she is gonna have to turn, cuz you know, the stroke on a 225 is 4.125 .............. and the pistons are only a tad bigger around than coffee-mugs.
As to
The engine in the linked article, it has pretty much only two stock parts left, the block and the head castings. At 13.25 Scr and a 270* cam, the bottom end below 3200 will be about as strong as a 5.2M. But where are you gonna buy gas to support the 235psi cranking cylinder pressure? IDK either; Fogedabowdit.

Static compression ratio of 13.25:1.
Ica of 64* At sealevel
Effective stroke is 3.23 inches.
Your dynamic compression ratio is 10.59:1 .
Your dynamic cranking pressure is 235 PSI.
V/P (Volume to Pressure Index) is 126