225 slant 6 or 360 v8

Ok, 3.73s
So now the picture is starting to come together
Given the 300hp, and assuming a 3150#raceweight this makes 300/3150 = a P/W of .095. Bulletin #40 says this,with race suspension and an auto should go about 12.5/12.6 in the qtr and about 101mph. Dividing the qtr ET by 1.57( the accepted conversion factor), we get 7.99 as an approximate 1/8M ET . Adding up to .5 for suspension problems you could indeed have a 300 hp slanty.
So say 8.25 seconds. This is about high 80s mph. With 3.73s and 27" tires, this makes about 6000rpm, at the top of second gear.(Hitting third, will drag the Rs down somewhere near 4200)
Now, here's what you have to know; To do this, that poor slanty, in addition to the already mentioned headwork,and the typical bolt-ons,will need a heck of a cam, and a heck of a TC. These heck-ofs are not real street-friendly.

IMO, you would be far better off starting off with an additional 100 to 135 SBM cubic inches. Then you can run a much more street-friendly cam and TC, and rear-gears, and still go 8s. A 225ish* cam and a 2800 should get you there, perhaps even with 3.55s with the right headwork.
The 3.55 gears with a 2800; I can tell you from years of experience with all kinds of gears,are a lot of street-fun,even with a 360-2bbl cam(252*), and are still fun ( but won't hit 8s)with a stock teener(240*) cam. Like said earlier, with a 2.74 low tranny,3.55s really shine.
Whatever you decide, all the best to you.