You have a total of 592 gravity points (74*8 ). At 6 gallons of final volume that's going to give you a gravity of 1.098 (592/6). If you use three and a half gallons of that with 1.5 gallons of top off water that'll give you an OG of 1.069.
But then you have lots of tasty wort left over. If you want to get down closer to your desired gravity you're either going to have to make more beer or toss some wort. Or mix up another batch with the remaining wort and some additional top up water, provided you have yeast you can pitch into it.
Also, your hop utilization will be lower with the more concentrated wort.