You guys are totally right with "better heads equals more power", but what is "better"? You are assuming that a pair of heads, like VNs, that are designed to flow and make good peak torque, as they do, and were designed to do at around 3500 rpm are going to work in a totally different application. Not a car similar to a VN that weighs just a little bit more, but a van that weighes twice the weight
empty, and I dare say is capable of easily carrying another ton, or maybe two. Potentially 5 tons.. ?
So, if you build a ~5ltr with VN heads and you capitalize on where those heads are designed to, and do make peak torque, which is ~3500rpm -give or take, then you are restricted to a cam choice that matches those heads as a "package" and is also designed to make peak torque at ~3500rpm. Sure, you can make changes to the cam's LSA and so on, but you're still stuck in the 3500rpm range regardless of what you do because the cross sectional area of the VN's ports are dictating the speed of the air moving through them which is controlling VE and where VE is -based on the size of the engine.
They are less efficient at lower speeds, and become restrictive at higher speeds, hense why they make peak torque at ~3500 which is always max VE, and no cam or intake manifold can change that very much at all, unless you create a mis-match of components.
Now, let's pretend that you make VN-headed 308 and it's makes a very respectable 400ft/lbs @ 3500 and you put that engine in a 3 ton (plus) van. That cam, although small by performance car standards, still has way too much exhaust reversion and overlap at off-idle speeds, which is bleeding-off too much cylinder pressure (Read: VE) to make enough torque to actually get itself and that massive weight holding it back to a speed where it can start performing. The engine and vehicle are a total mis-match and that there is the whole point of where torque needs to be made.
Ever driven a truck with a tacho? It's all low-down for a reason.
You need as much torque as possible, right off-idle, as soon as you crack the throttle open so the engine can pull with loads of torque, immediately.
No turbo will fix that. How are you going to get the turbo spinning at off-idle speeds with an overlapping cam, and you'll blow any boost you make though that overlap anyway.
Or are you going to put 3500 hi-stall in the van and boil the transmission in order to get the engine revving where it needs to be?
Or ride the clutch until it catches fire in crazy efforts to get these heads and cam at their working speeds from a standing start?
Like the HT502 peanut-port example, that is done with cylinder heads with a very small cross sectional port area in order to produce the required amount of air speed at where this engine makes peak torque. The cam it has purely capitalizes on that 2800 rpm, and with little-to-no overlap it has heaps of torque right off idle. So, at 1750rpm, this thing still makes a huge 490ft/lbs - only 20ft/lb less than it's maximum.
If just a cam and comp change were to make this 502 the torque monster that it is, that's all GM would have done using regular heads.
308 heads may be considered too small by performance car standards, but for good for a heavy truck, except you still need more cubes to get the MxTq/VE where you need it..
Very roughly with basic math, 308 heads on a 355 short would bring the torque peak down from ~2800 to ~2400.
With VN heads, even if you could stroke it to 400 cubes, the rpm where they make their peak torque would still be as high as ~2700.
TORQUE.