This is a long post, but I've mostly ran a combination nobody has. Real interesting.
-300, 85 f150, 4 inch lift, 4x4, np435, 3.55 gears, 35inch mud terrains, 2100rpm@62mph
-h519p cheap hyper pistons
-91 octane required (any less it won't shut off)
-estimated 8.8 static compression
-Stock rods, bolts, mains, crank all that stuff
-Stock 84 head (with screw in studs, Clifford springs, stamped steel rockers, milled 0.060)
-Clifford shorty headers, true dual exhaust with magnalow mufflers you can see through
-Clifford intake
Comp 268 cam installed with comp gears dot to dot
-Street demon 625 cfm
-Stock duraspark 2 ignition
The truck is daily driven here in Canada and does a bit of everything. Towing 7500lbs, bogging through ponds, heavy summer traffic, 20hr drives even plowing over 4ft snowbanks. I have a wideband AFR meter to tune it, keep that in mind. Its with me everyday and my only ride to town regardless of temperature outside from 85*F to -40*F.
The carb is a 625 cfm street demon carb with a tuning kit (78-82 primary jets, 7 or so metering rod sets, 5 springs, all for the primaries). The primary side of the carb is tiny, about 1/3 of the total cfm and uses triple stack boosters giving it incredible atomization velocity and response. The mechanical secondaries are huge and tuned with just jets. Now to the tuning....
With the smallest 78 main jets, the second leanest of about 7 metering rods and second leanest of 5 springs on the primary side. The secondaries are way lean about 8 jest smaller than out of the box.
It cruises at 16.5:1
Very light acceleration about 14:1
Fully open~200cfm primaries 12.5:1
Full throttle above 2000rpm 12.5:1
Now for drivability. I have trouble with the electric choke not closing fully in Canada's January cold (linkage jamming or the spring not moving), but with the choke fully closed it starts instantly (you know its cold when the oil pressure doesn't come up to full for about 20seconds and it starts driving in neutral). Despite the 268 cam, the tiny primaries and triple stack boosters makes throttle response faster than you can smack the pedal and pulls harder than the stock 300 everywhere above 1000rpm. Below 2000rpm the secondaries will drown it but above 2000rpm it twists the truck and pulls hard. When I let off the gas in third at 4500 the truck shifts and the shifters move about 6 inches, it pullllls. I never really measure milage cause I drive it like a lunatic, but I've measured 14L/100km (16.8 USMPG) towing a 2200lb car trailer at 60mph for 4 hours of highway. Not bad for a 4 inch lift and 35s.
Long write up, but I'm very happy with this off setup. I tried a 500cfm summit carb (the primaries sucked and it had several problems). The 625 cfm carb is a bit overkill and I would not recommend it unless you have a wideband AFR meter and patience, but everyone that drives it or comes for a ride is stunned.
Next post will be a picture of the truck (two different paint jobs), link to the build thread and the odd carburetor NOTICE THE SMALL PRIMARIES THEY ARE GREAT ON A DAILY DRIVEN 300. Hopefully I will get my hands on one of gramps SR heads to play with and chassis dyno.