Tatman, welcome to the forum discussions, I'm excited to see the I-H participation! My cousin had a '73 1210 4x4 in the mid-'70s, what a terrific truck.
I replied at your wheel size inquiry (biggman100's posting on the Roadkill database). Now I have the bigger picture around your truck and am happy to elaborate...
As with any I-H, the Line Ticket sheet is gold. If you have that still, you can part your truck down to the assembly line build components. The VIN can also turn up information. Years back, you could get a duplicate Line Ticket by approaching International-Harvester with the VIN. (Worth a shot, even today.) As a point worth making, the I-H trucks were built with the best components available in the industry. 1973-up 3/4-ton trucks like yours are often a prize!
Some quick questions and points to narrow down the truck's equipment:
1) Is the front axle a closed-knuckle with drum brakes? Disc brakes and an open knuckle front axle were optioned prior to your model year, and your truck should be open knuckle with power disc front brakes.
2) Which GVWR rating (see the door sticker): 6300#, 7500# or 8200#? This will help you identify parts and the load capacity of the axles and springs.
3) If a regular cab with 8' bed, you're likely on the popular 131" wheelbase.
4) Your Dana 60 rear axle is likely 4.10 or 4.56 ratio, the 3.73 was typically for 3-speed automatic models.
5) Does the engine have a four-barrel (original equipment Holley with a stock air cleaner)? Does the manifold look "stock"?
6) You should have an I.D. plate on the transmission, should be an NP435 four-speed, there should be a round I.D. plate on the transmission.
This is a great powertrain, the NP435 typically has a 6.69:1 compound low gear ratio. Your iron case NP205 gear drive transfer case is an all-time benchmark for superior OEM quality. This is the ultimate, and coupled to a 345 or 392 V-8 in good condition, should last a very long time.
There is the one-off use of an AMC 401 V-8, designated "400" in I-H 1973-74 applications. (I've only seen these in Travelall models trying to meet emissions compliance.) Your 49-State 1210 truck should have the bulletproof, industrial strength V-345 or V-392 I-H engine. Provide me with the carburetor's "list number", begins with an "R", and if the original carburetor, we can narrow this engine down. There are subtle differences between the 345 and 392, 345s are typically 2-barrel unless retrofitted with a four-barrel manifold.
Regarding your chassis lift plans, you're somewhat in luck here. The front springs are leaf type and can be re-arched or modified by a quality spring shop. You can use other period 3/4-ton trucks with your spring arrangement as a prototype.
There are rear spring spacer blocks available in different widths and heights, also U-bolts and hardware. With some creative measuring and comparisons, you should be able to build your own "lift kit" from aftermarket products available for other 4x4 trucks of the leaf spring era. If you're really lucky, you will find front springs with similar width, length and eye diameters in an existing kit. Shock absorbers require proper length increase to match the spring and block heights...We can discuss this further.
As for wheels, if you chassis lift, you also want to widen the track width to maintain a safe center-of-gravity. In this case, the backspacing and rim width will be determined by the proper "scrub angle" for the tires. (See my discussion about front end alignment and caster angle at the topic Megatron posted.) Your wheel bolt circle and center hole are common for other 3/4-ton trucks of the era (U.S. measurements).
Ford and Dodge each used the Dana 60 rear axle with drum brakes, and a Dana 44 front axle was a Ford F250 approach—with two wheel hub center hole sizes, depending upon Ford's front axle load rating. (Ford F250 went to disc front brakes later than I-H, to I-H and G.M.'s credit.) You will likely discover a similar OEM backspacing on wheels built for Ford, Dodge or even G.M. trucks that use front wheel hubs close to the design of your 1210.
A local tire shop can confirm your 1210's wheel bolt circle, the stock wheel backspacing and any clearance issues. This will turn out common to other domestic 4x4 trucks of this chassis size with Dana axles and similar brakes...I'm glad to assist further with your search. Please share your findings, others will benefit!
When you lift, you will also need a dropped steering pitman arm. Since I-H sought out the best steering available, your truck has a Saginaw steering gear. You will find this used on Ford and GM trucks, and if you match your steering linkage design to the right truck, you will very likely discover a suitable, dropped pitman arm for that application that will work on your steering gear and with the 1210's draglink joint. You may need a different draglink joint that will fit the pitman arm and also your I-H draglink sleeve. We can discuss this further, too. Safety is primary! You'll likely want a decent steering stabilizer shock (universal mount) as well.
This is an exciting and durable truck to own and build for your purposes, tatman! Expect my assistance and data to be available, along with input from I-H and Scout owners. Welcome!