You'd be best off with 380 flywheel hp on the engine dyno with the exhaust system attached.
300 rwhp needs about 380 hp net to make the target with a T5 manual gearbox.
I'm into GT40's and Panteras, and 351C 4V's. The history on these is clear.
Factory 300 rwhp ratings with just the 289 actually happend in 1967.
If you all (in my words) are arguing about anything else, you need councelling, IMHO.
The 351C 4V's were doing 300 rwhp in late 1970, when the 310 and 300 degree cam was added to the Australian export engines.
The 1972 290 degree hydraulic cammed Panteras were making almost 300 rwhp.
The time between the early 70's and emissions era of the OBDII era had Saleen looking at how much power you could make with the air flow restricted stock Cobra style intake. With a 5.8 engine, alloy Edelbrock heads, and a clean bill of Federal Motor Vehicle emissions, it was 371 hp net at 5100 rpm.
The S-351s started with '94 Lightning 5.8 short-blocks, to which Saleen added Edelbrock aluminum heads, 30-pound injectors, a roller camshaft and lifters, a 77mm mass-air sensor and 65mm throttle body. The final tally was 371-rated horsepower at 5,100 rpm and 422 lb-ft of torque at 3,500
And that sure looks like the stock GT40 style intake.
That engine almost heals over past 5000 rpm, and the 1.294" taller deck and lower EFI intake does its best to enusre the upper intake isn't a pinch point to power.
It took about 33 years for the 5 liter Windsor with iron GT40P heads to make more power than the 1969 Boss 302 engines. They never did it factory, but they sure can with the right cam and upper and lower intake, MAF, injectors and a little fettling.
Going back to 5 liter OHV engines.
300 rwhp is easy with just GT40P heads. de Tomaso almost did it with the first European Emissions 305 hp Pantera Si in 1991, but it got downgraded to 248 hp the next year..
The early small block Ford GT40 289's certainly did the 300 rwhp thing.
So did the 351 Saleen with alloy heads. So did the stock iron Explorer headed T3 Falcon and Falcon Pursuit 250.
Doing 300 rwhp is as easy as finding the right intake, cam, MAF and injectors.
If you want 300 rwhp, you follow the 5.6 liter T3 Tickford build list, and don't add the stroker crank. Longer rods and short deck pistons is what I'd use.
You can keep the Explorer GT40P heads, but you
need the Upper and lower intake, MAF and injectors. This is because the air flow of even the Showa GT40 upper intake is lousy.
An upper and lower intake like this
MAF was the 4.6 liter Quad Cams 82 mm item
Mild porting, the right valve springs, rockers and making sure the EECV system isn't running lean will help.
That 335 flywheel hp and 369 lb-ft was with a 5.6 liter engine reving to 5250 rpm for power, but able to take 6000 rpm.
It was just the tip of the ice berg for that engine, and Ford Australia downgraded to non alloy heads after a run of 500 alloy headed 5.0 liter engines.
For any EFi pushrod 302, you just need more cam, and an EECV flash, or a later EECIV re-calibration to do it.
In terms of what even a 295 hp 2002 5 liter Windsor OHV V8 is, (a 40 year old rework of a 289), the engine gave an exceptional account of itself against the same years 5.7 liter LS1's. The T3 335 hp 5.6 stroker was in another catagory. Meaner, harder edged, not as smooth, but still able to take years of 4000 pound luxo barge punishment with reworked stock 'Exploder' heads and blocks. It was all because Ford Australia had the last remains of Aussie Jac Nassers aborted work in allowing the Premier Automotive work on putting Aston Martin hand building engine bay process into stock cast iron thin wall blocks.
Ford Australia re built the US import 351C engines from 1970 to 1972 with QC engines, then stopped when Total Performance had a melt down.
For 1999 to 2002, the 1970-1972 QC process got re-started with Tickfords ex Aston Martin engineer Dave Flint, and then continued with the Quad Cam 5.4 in 2002 and then the later Quad Cam 5.0 engines untill 2016.
The 5.0 Explorer blocks were the low point in quality compared to the early blocks, but a little spit and polish, and you had an extra 120 hp with stock Ford Motor company parts.
The Aussie use of special EFI upper and lower intake air managment, the use of non forged pistons with long rods, cast cranks, and plateu hone bores to be kinder to the block at 6000 pm, all of it has filtered into current US practice.
Main bearing block girdles,
upper block ties, and the simplicity of using the junk yard Aussie EECV, or EMS computer systems when there isn't IM testing,
well, that opens the door to streetable 400 hp at 6500rpm 5.0 liter combinations which won't split blocks at 7000 rpm.
That's why I like 5.0 engine build ups like this.
Don't be scared of EECIV or EECV.
Its got some issues, but you can swap the whole system over if you have the pinouts, and its really nice if you have it working with 380 hp and not tripping a check engine light. Some Four eyes have MIL lights... the transfer of a broken scrapped SUV engine into an old Fox platform makes it a HSV...Humanitiran Service Vehicle.