Can I do this without having to tip the shaft or add an extension?

Thanks

you can blindbore - you can cut the shaft on the D2 and drill it out to about 1.2 inches, then install your onoff shaft.

if the head weighs the same on both, the flex would be the same, the only variable would be your head design.

pro shops should be able to blindbore for you. let us know how it goes