Either 70-99-70 or 99-99-70.
Whilst PKing, strength should be your priority stat followed by attack. Obviously for staking, swap that strength with defence
Well if you are too scared to go PKing, a 99-99-70 is a poor choice for staking.
For staking- Prioritize Attack and Def
For PKing - Prioritize Attack and Str
My recommendation for you would be to get 99-93-99 with as low prayer as possible. This gives you the ability to PvM and do Bossing and such, but also will leave you lower combat than your counterparts and will allow you to box.
Since you do have 70 prayer, you can also consider just going 99-99-99 as some people you box will be 99-99-99 although DEF and ATK are MUCH more important for staking (boxing)