Mid-70's in the shade would be near 90's in the sun. Lettuce and other veg's you mentioned grow happiest in temp of 50's~70's. You might be able to grow fast maturing lettuce as mesclun/baby lettuce in the next 30~40 days in the shade if the 70's in the shade temp hold up.
Otherwise, you'd be better off either growing indoors under lights with A/C kept below 80Ã‚Âº (if you keep your house that cool) or wait until fall. Lettuce can survive mid 20's and light frost. Planting in the fall is a good idea. I'll let your local experts advise on the best time.