Very nice truck. Really clean and straight. Yes it's the first series 1965 which has a 122" wheelbase instead of a 128. That 6" of wheelbase makes a difference in towing. The 128 is more stable.
The in-cab fuel tank has to go. There is a better place for a fuel tank, like in the rear where the spare tire might go. There are also spaces along the framerails that might be useful.

Even though the truck is a stock color, which was from time to time called Turf Green, duffers will try to convince you the truck is Forest Service Green. But what do they know?

Here are some sore spots on this cab style:
1. Fuel tank, already mentioned. If it leaks or cracks, you're swimming in gasoline.
2. Door latches. These are a major pain in the bum. They have an aluminum star wheel and it is not reproduced. These are like car latches from the fin cars. Vintage Power Wagons used to reproduce them, but not for 10 years or more. When my star wheel wore out I had to cobble a workable latch from a rear drivers side latch from around a '59 car.
3. The hood hinges wear out and the hood sticks up at the rear. Nice thing about the hood is that it opens up wide enough that you can pull the engine without removing the hood. If your hood hinges work OK, keep them lubed.
4. The truck bed is single wall. That means that any dent that gets put on the inside shows on the outside. I had a crankshaft roll and bang!
5. The tailgate is supported by chains which bang on the body in rough going.
6. Wheelwells are not in the usual places in the bed so a "regular" mat won't fit. That bed is an odd duck. 122" wheelbase strikes again!
7. I've had trouble with the taillights. Also no backup lights.
8. Fortunately the front turn signals are easy to find on Ebay.
9. The brake and clutch master cylinders are basically the same unit, with exception of the residual pressure valve on the brake master cylinder. It sure would be nice to have a dual braking system.
10. Good luck getting power steering.

Now on to the frame: Unless you are used to trucks of the '40s and '50s you will need to do a bunch of studying. This truck is set up with four point mounting for the engine/transmission. There are two mount points near the front like "normal". There are two mounting points on the bellhousing. The crossmember that the bellhousing mounts to is massive and sits right in the way of any non-custom header's collector.
The transmission hangs off the back of the bellhousing. Emergency brake is a drum on the transmission. The rear axle has no provisions for emergency brakes, at least not on the 1/2 ton.
The hydraulic clutch slave cylinder on the passenger's side of the bellhousing is directly in the way of any noncustom header tube.
The bellhousing wraps around the flywheel in such a way that you cannot remove it before removing the engine. It weighs about 72 lb. It should have bolt patterns for flathead 6, slant 6 and A engine.
You cannot pull the engine and transmission out of the front of the truck. You remove the transmission first, either through the cab or down to the ground, then the engine will come up and out the front.
The frame is set up for a front sump oil pan.

Now we get to the engine. The 318 Poly is NOT larger than an LA engine, physically. It just looks that way. It has the same deck height and bore spacing. Crankshafts are the same. That means that 340 stroker cranks just drop in. Rods are the same. Oil pump is the same. Distributor is the same. What is NOT the same is pistons, camshaft, cylinder heads and valvegear, intake manifold, exhaust manifolds. IF you like to feed the weirdness you can build a big Poly motor. But it will cost you big bucks. There are about five intake manifolds that are not 2-barrel. None of the intake manifolds is currently available for less than $500.There is the Edelbrock P-600 3-2s, usually Stromberg base, the 2-4 barrel Fury manifolds, currently less expensive but take two carbs, the single 4-barrel stock intake, running around $600 last time I checked, the 7503 Weiand, not available new and usually getting around $500, and then the new one that currently retails for $670 or so, if they are actually making them. Camshafts are limited to regrinds except that Camcraft has a bunch of new cores, $185 apiece, but then you have to get the core ground, so I think we're looking at $300+ for a brand new cam. Regrinds of stock cams are not too bad at under $150.
Because the valves are in different places on a poly than on a Wedge, pistons are at least semi-custom. Sealed Power makes stock pistons in their antique line but I think they're about $400 a set.

Don't get me wrong, I like the Poly because it's weird. My engine was a stocker with Weiand 7503 and Holley 570 Street Avenger. I used a Direct Connection electronic distributor kit and finished off with dual exhaust. My truck got an honest 16mpg with a lot of rejetting of the carb because I live at altitude. The Poly heads are actually better than the 318 heads and the Poly in near stock trim has more power than a LA 318.

I swapped to a 1986 roller cam 318 and used the 360 hydraulic roller cam and a 1975 or so stock 360 four barrel intake with the 570 street avenger. I used the late model exhaust manifolds with the old exhaust system and picked up 6 mpg right off the bat. Put that in your Poly and smoke it!

If I was doing a ground-up reconstruction I'd look seriously at a GIII Hemi, take #200 off the front end and make steering easier.
A B/RB will bolt in and a B makes more sense for room especially as you can build them to 500 inches or more.
I didn't think the engine bay looked empty when I substituted the LA for the A.
One last thing,,,,,The NP420 transmission has only synchronizers in 3rd and 4th gears.

R.