The 70's were one of the lower points in horsepower ratings of the little Chevrolets. So, rather than a stock rebuild, do a standard mild performance rebuild. Accept the 350 in your thinking as a starter block and attack it from there. You can make an excellent driver out of it for just a paltry few more bucks than stock rebuild by being careful with your parts selection. You need to make the whole engine work together the best that it can. A properly built 350 will move that van right down the road with class and last forever. Look for horsepower output in the 325-375 range. Bottom end torque is what gets that van moving from a stop, build for that.