Hi Sam,
You should do a preliminary analysis for anchor locations through whatever initial data available and add some contigency factor. And arrive at anchor location.
Even in preliminary plot plan you may put a note covering yourself. And durind detailed stress nalysis you can confirm that.

To my understanding Piperacks are the first thing to constructed at onshore plant. So you should address as early as possible otherwise project ppl will be behind you.

And for similar type of plant anchor bays remain same because of process parameters remains same. So it will give an guidance during prelimary study. Offourse volume handling may differ and accordinly dia`s and thk.

In my opinion, during preliminary study you should not fix the anchor bay location, whatever pressure from project might be there.

It should be done one feed verification P&id`s and line list land on your table.

Hope this helps.
Regards