torquer SBF
You could get near or even over 400 HP with a 347, but it will take a bit of work, to do so normally aspirated.
As I mentioned, the 351W crankshaft, while having its mains on the same centers as the 302, has much larger journals, and it would require a LOT of machining to cut them down to the 302 journal size, the 302 journals are around 2.25 inch in diameter and the 351 journals are 3 inch. The 351W crank also has much beefier rod throws, and it would require a lot of machining inside the block to allow them to clear, so as it stands now, the 347 stroker crank, which has a 3.4 inch stroke is about as big as one can go without a lot of serious machine work. There are also other things to consider, such as rod length, piston deck heights etc. With the 347, youre at the upper most limits with those things now. The only other alternative with a 302 block would be to get an SVO block from the factory or after market that would allow you to go with a bore larger than 4.06". And you are back to spending a bunch of bucks then.
You dont have to bore out the 351W when you install a 3.85 stroker crank, however, when people build those engines like that, the engine usually needs a clean up bore job anyway, and that is why they advertize the engine as being a 393 with the stroker kit, as normally the engines takes a 30 over bore to clean it up. With the stock 4 inch bore, the engine displaces 387 cubic inches.
The 3.85 stroker cranks go for around $400- 500. That is the biggest single outlay of money in the operation, as you can use your stock 351W rods with it along with stock 302 pistons. So its a pretty straight up engine build and economical as well. Additional costs will be approximately $100- 150 for the pistons and another $100 or there abouts for your bearings, rings, and gaskets. As I mentioned previously, you can build a 3.85 stroker for under $1000, unless you want to put aftermarket rods and some super whiz bang pistons in it. I would recommend getting the entire rotating assembly balanced before installing it in the engine. This should run you about $150-200 at the most, and you will still come in at around $1000 total expense on that part of the engine build. Your cam, valve springs, and lifters would run you another $200, approximately. Since you arent building a super high RPM engine, go with the recommended springs your cam grinder goes with. I would also install a high volumn oil pump while youre at it.
As for your heads, Take of a rocker cover and see what kinds of markings are on the head in the valve spring well.
Here is a samle of realistic engine setup for street in a 351W. I have a 351W in an 88 F-150. Engine mods include, 40 overbore, flat top 9.4:1 compression pistons, with TFS Twisted Wedge heads out of the box, custom ground Camresearch solid lifter 230/235 degree @ .050". .510/.515 lift 106 degree lobe separation, Edelbrock performer intake, 600 CFM 1850 series Holley carb body mounted on an 83310 series 750 CFM base plate,Mallory ignition, 1 5/8ths primary tube headers with 3 inch collectors.
This engine dynoed in real life at 398 HP @5500 RPM and delivers over 400 LB Ft Tq from 2500 to 4500 RPM. This should give you some good perameters to work from. This is a torque engine, and I shift at max RPM of 6000. I have played with this engine on my pc dyno, and with the 3.85 stroker kit, the engine would still have the same peak RPM but the torque figures below that are abundantly higher, well into the 400 Lb Ft range. Dont take this as meaning that you couldnt get a lot more torque and HP out of a similar setup.
Lift and duration of your cam govern how high the valve will lift and for what length of time the valve will be open. Generally high torque cams will have a high lift with not so great a duration. As one guy told me many years ago, lift helps torque and duration gives you RPM. The higher a valve is lifted and or the longer the valve is open, the more fuel and air the cylinder is capable of drawing in. So any modifications in either area are going to allow the engine to take in larger amounts of fuel on each intake stroke. That being said, its not uncommon, though it doesnt actually make sense, that an engine with a cam thats maybe 10-15 degrees larger than stock, along with the coordinated lift, will often give better gas mileage than the stock cam, with all else being equal or the same as was when the engine was stock.
At one time I had a 70 Ranchero with a 302. Before I did any mods to the engine, it got about 15-16 MPG. After putting a Crane HMV 260 cam, 1 1/2 inch primary tube headers with 2 1/2 inch collectors, an old 66 289 Hi Po iron intake and 480 CFM Ford 4 bbl on it, I was getting 17-18 MPG on the highway. This was with the stock 3.25 gears and C-4 tranny. On the cam, I went from a stock 184/189 duration with .368/.381 lift to a 204/216 duration and .456/.484 lift.
As for your spark plugs, it sounds like you dont have the origional heads on your engine, which is why you are having trouble with getting the proper plugs. On stock heads, Ford has used only two basic plugs. So it sounds like your engine could have aftermarket heads on it. My Twisted Wedge heads dont use a plug that is anywhere near what the origional factory plugs would have been for the 78 heads that were origionally on my 351W. For some reason, it seems that a lot of the aftermarket heads will use a plug that would have come out of a Chevy engine, and being that most aftermarket heads are using Chevy valves, even for the Ford engines, that wouldnt surprise me at all.
I would also think that somewhere on the ends of your heads there would be some kind of identification as to the manufacturer, if they arent stock Ford heads. Find out what kind of heads you are running, and if they are aftermarket, get ahold of someone that sells those heads and find out what the proper plug should be.