Running an MT4 EA on a custom timeframe

 
Hi i want to run my EA on a custom timeframe chart of 8 hours, by using it in combination with the period_convertor_opt indicator. I have attached the period_convertor_opt indicator to my 1 hour chart, and want to run my EA on 8 hour charts. I have set the period_convertor_opt indicator multiplier to 8. I have opened my H8 offline chart. The trouble is, i now do not have the option within the tester window of setting the period to H8. I am presented with the default M1, M5, M15, H1, H4, & DAILY timeframe periods. Is it possible to run an EA/backtest/optimise an EA on a custom timeframe? If so, how? Many thanks.