Limb length has absolutely nothing to do with strength of the bow or the power it produces. I will guarantee in writing you cannot get full draw of the sinew backed osage Sioux horsebow in my shop. It's 36" long, has an 18" draw that pulls almost 90#'s. Heck, I can't pull it, I black out trying to string it!
Generally speaking, longer limbs need longer draws to actually get efficiency from them. In order to get a "load" on the back of the bow with longer limbs they will need to be thicker. By adding mass to the limbs you actually slow them down. In order to counter this issue of heavy and slow limbs you will need to make them lighter by making them much narrower. That's how the classic English longbow (ELB) works with such deadly force.
It's all about trade-offs and what you are wanting.
Ok, go 67" tip to tip. Narrow the bow down to about 1 1/8" at the handle and bring the tips down to less than half an inch. Keep it rectangular in cross section. You can allow the grip area to be a little stiff now, making more of an elliptical tiller. Being a longer bow it will probably shoot more consistently, too!