I get the impression from your post that perhaps you're not familiar with the terms "backspace" and "offset". Those are 2 different dimensions on a wheel, with offset being dependent on both the backspacing and wheel width.
Backspacing is the dimension from the wheel mounting surface (WMS) to the inside edge of the wheel flange on the backside of the wheel. This is a critical dimension, as it determines clearances in relation to the suspension, steering, and brake components. Offset is the dimension from the wheel centerline to the WMS, usually measured in millimeters. The backspace and wheel width both have an effect on this dimension. A wheel that has the same amount of backspace and front space would have a 0 offset. If the wheel has more backspace than front space, then it is Positive offset, and if the front space is great than backspace, that is Negative offset.
Most wheels for the vintage trucks we deal with use either Negative or 0 offset, and average 3 to 3 1/2" backspace. Modern trucks, mainly due to the suspension designs, tend to use Positive offset wheels. This is mainly in an effort to package IFS 4x4 systems while not increasing track width, but also to get better handling getting the centerline of the balljoint as close to the centerline of the wheel.
That said, ideally, you should really try to see if the wheel comes in the correct backspaceing for your truck, which as mentioned, should be around 3 1/2". The offset will vary depending on what width you're considering. If you must use a spacer, use the type that bolts to your hub and has it's own separate set of wheel studs that the wheel actually mounts to. Those are much safer that using an excessively thick spacer.
A few words of caution, using a 20" wheel is going to increase not only the rolling mass of the wheel, but also unsprung weight. Both can lead to handling and ride issues if not addressed. You'll want make sure the suspension system is in 100% working order, and preferably with upgraded shock to make up for the unsprung weight. Even more critical is going to be the brake system. The stock drums will not be adequate. At minimum, power assist should be added, and realistically, you should consider a power assisted disk brake upgrade with at least a 12" or better rotor. Another word of caution with wheel widths and backspace/offset; If you get too or too little offset, that will change the scrub radius angle. This is a critical handling angle, and while there is some leeway, going to far out can cause handling issues.
