As far as running a 21 spline with a 4.7 in the crawl box, it should make no difference if that is 21 or 23 spline. It has to be whatever your transmission is to match the female in inside your transmission. You're not applying anymore torque to the input shaft on the front of the crawl box by changing the gear ratio inside, it will always be the amount torque coming from the trans being applied to that part. It is after the crawl box where that torque is multiplied, so you would be putting double the stress on the 23 spline input between the two cases. There is a much higher risk of breaking that 23 spline with that much torque going to it, which is why it is suggested to run the 4.7 in the back. But its been done plenty of times with no problems. That's why marlin makes his total spline input with no neck down spot like other companies.
