Pre metric British cars use British standard nuts and bolts, which while similar to the US standards are different.
British Standard Fine (BSF) is very similar to American Unified Fine (UNF), differing only in the thread pitch. BSF threads have an Angle of thread of 55 degrees while the UNF threads have an angle is 60 degrees. Both standards use the same wrench sizes and the same size clearance holes. Mixing nuts and bolts will result in a considerable loss of holding force, fatigue resistance, and strength. While they look the same and use the same wrench size, they are not the same. Use a BSF tap and die set to clean up threads and when you need to replace fixings be sure to order replacements of the correct standard or replace both the nut and bolt.
British Standard Whitworth (BSW) and American Unified Coarse (ANC) are very different coarse thread standards that are not at all compatible. Sir Joseph Whitworth proposed the Whitworth standard in 1841and like the later BSF standard has a thread angle of 55 degrees. Where people get confused with wrench sizes is that Whitworth wrenches are given by their screw thread diameter. More modern standards size wrenches by distance across the bolt heads or the nuts. Whitworth wrench and socket sizes are marked with a 'W' suffix behind the size. So a 3/8" Whitworth wrench is a lot larger than a 3/8" BSF, UNF, or ANC wrench. The change in how wrench sizes were calculated came as a effort to save metal after better metallurgy was available making stronger wrenches.
To confuse things even more, in 1919 Morris Motors took over the French Hotchkiss engine works and moved the company to Coventry. The Hotickiss tooling was metric but metric tools were not common in the UK at the time so Morris came up with fixings that had metric threads with bolt heads and and nuts dimensioned for Whitworth tools. So both Morris and MG engines were built with fixings that had metric threads and Whitworth size bolt heads and nuts between 1923 and 1955. So if you have an early Morris or MG in your garage be careful not to mix their engine fixings with any Whitworth fixings you may have.
An American source for British tools and fixings
The drive train of pre-metric Series Land Rovers use Whitworth bolts and nuts. You can mostly fake having the right size tools with a combination of Metric, AF wrenches and an adjustable spanner. However you risk damaging the heads with improper fitting tools. You are best off having the correct size wrench or socket for the job you are doing.
You do not need a complete socket or wrench set for your truck as there are some sizes that do not exist on your truck. The most common Whitworth sizes for your truck are:
3/16 - The little nuts holding the transfercase bottom plate and I think the retaining bolts for the big swivel housing seal
1/4 - Common use
5/16 - Common use on the transfercase. You will need a 5/16ths socket for the nuts inside the transfercase holding it to the gearbox
3/8 - The transfercase mounting bracket fixings and a couple other places
You might also consider a 7/16 and a 1/2 spanner. They are not as commonly used as the other sizes and you can normally get an adjustable spanner in where these nuts are located.
Additional info from Geoff Tobin:
Land Rover transitioned for BSF to UNF to Metric at various times and not consistently. The propeller Shaft bolts other than those on the Transmission Brake drum on II, IIA, and III vehicles were always 3/8 UNF. Transmission brake BSF Differential retaining bolts are listed in the IIA parts catalog as BSF or UNF (check for alternatives). I believe that transition may have been about the time of the IIA Suffix B. Hub Drive Flange Bolts remained BSF until Metrified. Wheel studs were 9/16 BSF thru IIA suffix G. From IIA Suffix H the thread is M16 x 1.5. There are 9/16 BSF lug nuts with 1 1/16 across the flats (576103). Series production went Metric in about 1978. Range Rovers, 101s and the SD1 were ground up Metric designs. The only UNF/UNC being found on the 3.5 V8 engine (American tooling). Very few if any other legacy parts were used in their designs. I suspect only the wheel lug bolt circle on the Range Rover and 101.