I remember back in the 1968-70 years how the 426 Street Hemi always had plug fouling issues after some street miles where the 440 could charge on without issues. Was this due to the center plug location of the Hemi combustion chamber?
On our 1970 Hemicuda I remember when the plugs had been in a while the first time or two when you hammered it you would hear that top end misfire and fresh plugs would clear it up. Don't think the Hemis of that time were too rich at cruise due to the lean jetting needed for emissions and the
Street Hemi camshaft really wasn't that big in duration/overlap for an engine that size. Ignitions on both engines about same as far as spark goes. One of first things we did was install a CD ignition box on our car which helped a lot. Do remember having a cigar box full of used plugs out of the Hemi back in those days.

When I was in high school my best friend had a 1970 RR with 440+6. We had to change the spark plugs in his car every Friday before going cruising. It is a bit of a big expense for us back then! I had a 426 with a factory cross ram in my car but I didn't have to change the plugs. My carbs were not as rich as his. Neither one of knew how to fix carbs back then, but we did know how to change the plugs.

I think part of the problem was intake manifold gaskets not sealing well. You had to keep retorquing them on a regular basis. Also you had a dual point distributor and the points needed regular maintenance. Looser piston and valve guide clearance didn't help either. BUT if you kept everything up to snuff they were really strong running motors.

Yep when you had one tuned up with fresh plugs it would pull hard up to 5800-6000 with the factory hyd. cam.

In the mid 70's after only 10 miles of driving in town my hemicuda would load up. Very annoying, so you would have to drive around the "loop" outside of town and drive it wide open to clean it out. (Not that I minded.) I was a teenager, I didn't know anything. I would clean the points with a thing called a flexstone that I got from the Snap-On man (wish I knew where I could get one of those now) and that didn't help. I sure wasn't going to work on those AFB's for fear that it would never run again. Electronic ignition was the greatest mod ever for a Street Hemi. I don't think I've ever heard one miss since. Give the inventor of that the Nobel Prize. It changed everything.

Electronic ignition and an MSD 6C really helped plug life in my old Street Hemi. At least that's how I remember it.

I drove my 67 Hemi GTX convert on two drag weeks, ran mid-13s, and have never pulled a plug since I bought the car. It is still running the stock dual point distributor and stock coil. I don't know why some people have problems with fouling plugs on hemis. I never do.

My hot-street Street Hemi was initially passing some oil through the guides, which we cured with better silicone seals.
Initial carb setup was quite fat, fixed that with some stagger jetting and springs (cam didn't make much vacuum).
MSD6 & electronic distributor were there from the start, but swapping coils from Blaster2 to the Ford F150 improved everything including starting.
Plug life was good after the seals & carb work.
Always seemed to me that the stock valvetrain design, plus the OEM rocker-shaft springs, imparts side-loads to the valves.
Intake design seemed to make for goofy fuel distribution except for WOT.
I always made sure that every drive had some WOT time, just for scientific purposes

426 Street Hemiroid motors are sensitive to the mixtures and quality of the fuel you're trying to use, changing the rear carb metering rods to the next size leaner usually help keep the plugs cleaner longer
The MSD race boxes were the best and simplest cure for rich running motors, wedges, hemi and SB Mopars

living the dream