Cool truck with an old skool LA 318. A modern magnum 318 or 360 will bolt right in place of an LA 318. Theres 4 barrel intakes out there that fit the magnum head pattern. Your original distributor will fit the magnum engine. Its kinda like MoPars version of a vortec. It was an evolution of a mopar small block. Your stock exhaust manifolds will bolt to a magnum, Magnums also have roller cams, If you decide on a 360 magnum, these engines are externally balanced, and will require an externally balanced flex plate so the engine doesnt shake itself apart. You will find them in ram pickups until the Hemi came out. They continued on in the B series vans until 2002. Magnums use SAE bolts. Nothing on em is metric.
Transmissions, Speaking of which, if you find a 1991-1995 360 automatic, you want the trans too. It's a 46RH or hydraulically shifted. Basically its an A727 big block gut trans with a lock up torque converter and an overdrive. Chrysler OD transmissions do not have an extra detent like GM ones. When OD and lockup are turned off, it's a regular 3 speed. You use toggle switches to turn on OD and lockup above 55mph, then turn them off below 55 mph. Theres also kits when retrofitting this trans to other vehicles you can buy that wire up pressure switches to automatically lock/unlock the trans while driving for you that have a toggle so you can turn it off for tow/haul.
You can use a mopar electronic distributor and fire it with a GM HEI 4 pin module. Check out
www.designed2drive.com for a nice 4 pin module bracket for a chrysler electronic distributor. With this you get killer hot spark, and you ditch the firewall ballast resistor and wire the 2 wires together.
Also on the wiring deal, Chrysler's run the alternator output power through the firewall bulkhead to the ammeter, then to the battery. I recommend you bypass the ammeter and reroute the alternator output directly to the battery with an inline fuse link. See pix below for wiring ideas to help you out. Pic 1 is typical how that truck would be wired. Pic 2 is how to bypass the ammeter. Disconnect the battery before cooling with the dash as those ammeter leads are constant hot with battery connected. I just use a bolt and nut and a piece of shrink tube to bypass the gage. Final pic is the GM 4 pin module running a chrysler electronic distributor.
My son and I are building a hot 318 for his car. Got a free 1977 318 short block as a basis for it. Those are great engines that most regard as boat anchors. They have a generous 3.31 rod ratio, basically the same stroke as a 340, just not the bore size. What they lack from the factory is good compression, good camshafts, and are hobbled by 2 barrel carbs. The teener we are building the stock pistons TDC'd at .080" in the hole lol. This engine will have a balanced forged bottom end, zero deck keith black pistons, 65CC closed chambered fast burn heads off an 85 chrysler fifth ave with larger valves, and port and bowl work, off the shelf Melling 68 340 cam, weiand stealth intake, and an eddy 600. Engine should be good for 300-325hp and rev to 6K. If my math is correct it should end up with around 9.7-1 CR. Stock mid 70s teeners are around 8.0-1 and will run on moose piss lol. This engine should idle well with good vacuum. So much for a boat anchor.
I hope this all helps you out.