You just need one special and it's cheap - or at least they used to be. Used to cost $10 but haven't had to buy a new one in many years.
It is an injector height gauge.
Your Detroit has a fuel injector pump for each cylinder and each is activated by a rocker arm - just as the valves are. The "rack" is a general reference to the arms and linkage you have to adjust. With the injectors - you use a small gauge that sets the proper depth of each injector. Depending on the engine and the injectors used - the gauge you need can vary. if your engine is all original, your manual should give you the gauge number. If not -you can go by the number on the fuel-injector-pump.
If you can't get a gauge - you can get that height spec and make your own.
Gauge looks like this - and they very by height.