Hi Damo
I am in NZ currently restoring a '47 1 1/2 tonne. Our trucks will be the same- maybe yours has slightly linger wheelbase. I assume your truck is RHD and would have been imported fully built from Canada. You can call it a 4100 or 4400 which are the closest US models.
1- Springs. As you have no clearance, they will be shagged out or snapped at the rear shackle. Car ones fit but are much softer and mine had a car one on one side. You will find they are cheap to get made. If you are on a budget, old toyota corolla wagon leaves can be inserted, same profile and curve. Get the specs and full service manual from oldcarmaualproject
http://chevy.oldcarmanualproject.com/http://chevy.oldcarmanualproject.com/shop/1948_51truck/index.htmYou will see that your truck should have 9 leaves in the front springs. Which I see in the photo.
2- Lowering- leave it, its a truck. The centre of braking force (the spindle) is in line with the axle mount to the spring, to minise twisting force leverage into the spring. Lowering will upset this. And as stated above, the drag link wants to pivot around the pitman arm, to minimise bump steer which is strong on these trucks.
3- Brakes. I'm not sure who your engineer is, is that a mate or a legal requirement for modification? I dont know how the regulation works over there. You will see that the brakes are crude but simple and colossal, and were made to stop the most overloaded truck. The simplest path is to replace everything as original, all the parts are available. See links below. Unless you intend to add a lot of power, the brakes should be fine (mind you I have not driven mine yet, the chassis/mechanicals are complete, just starting on the body).
4-Engine. Your engine, if it was a canadian truck, will be a 216 with cast iron pistons, poured big ends and splash lubrication, making 90 bhp and geared to cruise at 50 mph max. I see acorn nuts and a small sidecover on yours so I think its got this original engine. The easy upgrade is a 54-62 blue flame which you will find easy to get as there is always some hot rodder replacing one with a 350. But in Aussie I agree a Holden motor would be the go , a 202 with a 5 speed would look the part, although I think best fit and cost would be a BA Ford Falcon motor (sorry all you Chevy purists). Or a Holden V6.
5- Diff. Yes its the biggest diff ever, 12" crown wheel and yours will be either 5.4 or 6.1 ratio. With the big 20" wheels you will be doing 3000 rpm at 60 mph with the 5.4. If you add a 5 speed with .85 overdrive, you get down to about 2500 revs and in a truck that weighs 2100Kg on its own, thats probably a good setup.
I think maybe the same diff was made in a 4.5 ratio in the V8 1.5 ton pickups 1955 through 1960 but I need to establish this for sure. Right now this is just a hunch.
Parts-
I have purchased off thefillingstation and Jim Carter. No problems.
Also look up
http://www.truckandcarshop.com/trucks47_59/t47_content1.htmlAnd there are others.
You will be gobsmacked at how easy it is to get most stuff. As you delight in your high dollar (unlike the Kiwi $).
Warning- shipping to Aussie and NZ starts at US$45, even for a washer, so order quantities, and then be amazed at the escalating cost. Freight usually makes up 25-33% of my order costs. And above a given limit ($500 in NZ) you will be charged import duty.
All the best and you can PM me or my email's in my profile.
But you'll always get excellent help in the big bolt forum.
Cheers