In the reloading manual I have it shows a load that requires a magnum primer. If a load requires a regular primer can a magnum primer be used in its place?
After rereading your post and answering exactly what you requested, the answer would be no, you should not substitute! Reducing your load as robocop10mm has posted would be appropiate, but what would be the advantage?
Data that is published has a built in safety for users and also liability issues. However, the people that are publishing this data worked up the said loads using the components listed, and determined that the primer used was adequate for the application. Using a Magnum primer in the place of a Standard primer has no benefit if you are seeking the performance from the data which was published in the first place.
Magnum and Standard primers are used for a particular reason when looking at powder burning rates, and by substituting you could very well be in the danger zone without knowing it. There are loading programs, such as Quick load that show pressure zones when using different components, and if you do plan on changing any component other than was listed, then you must take into consideration the composition of the components you are changing. In retrospect, published data is there for a reason, not only as fact of their findings, but the needed safety involved when loading your own cartridges.