This thread speaks directly to my desires for the engine I plan to use in my 56 Docge pickup daily driver. I'm mounting the 56 cab/box onto my 2001 2WD Dakota chassis, and ditching the 4.7/5-spd for a built 5.2 Magnum port EFI , as I like the idea of a classic Mopar engine in a vintage pickup. The drivetrain will be the 9.25" 3:55 ratio limited slip rear axle from my Dakota, overdrive trans of about .67 or .62 and stock tire diameter to get on highway revs at about 1800 at 75 mph. I know from many trips driving my 2001 Dakota if I can cruise at 60 mph in 5th gear (.72 ratio) it keeps the RPM just under 2000 and I have recorded fuel mileage over 21mpg. Based on that, with a built 5.2 EFI Magnum producing over 350 ft-lb of torque in the 1600 to 2000 rpm range, I feel its possible to achive consistent 20mpg at 75 mph, and still enjoy over 350 rwhp when I want to stomp on it.
Gene has a 318 in his vintage pickup on a Dakota chassis and said he's gotten 20mpg on occassion. Thanks to Gene's suggestions I ditched the plans for a 4.7HO and will go with the classic build plans for a 5.2 Magnum EFI, Keith Black flat top pistons, deck the block, 5.9 heads, block hugger headers/dual Magnaflo exhaust, Edlebrock Air Gap intake manifold, MSD ignition and whatever else I can plan to create a vintage daily driver that can produce good fuel mileage for highway cruising.
Couple points for consideration. 5.2 and 5.9 magnum heads are the same, no difference so use whatever good set you have or can find.
If you are going to deck the block, you may not need to spend money for KB pistons as 5.2 pistons are closer to the deck than 5.9. If bores are good just deck the block to get the piston crown to zero deck or close.
Magnum manifolds would be fine if they fit and there would be very little if any improvement with block hugger headers. A regular performer intake manifold with a thermoquad( or second choice quadrajet) will net you the best mileage if you are switching over to carb.
Maybe be overkill, but if you are going all out on mileage an 8.25 rear would be worth a little over the 9.25. Tonneau cover and electric fan are also worth looking at.
