What's the data plate for that boat say about the max HP?
Both of my boats are around 18' and I'm sure they'd be extremely underpowered with a 70..... I have a 175 on one and a 140 on the other (both are rated around 180 HP maximum), I can feel the difference in acceleration (the 175 is significantly better), but kinda' like the 140 because it's much more fuel efficient.