Don't get too hung up on supporting the shaft. Yeah, putting another hammer up in there, or a block of wood cut just right is helpful, and not a bad thing. But it's not likely you will hurt the shaft. You would have to beat pretty hard to bend/damage it.
You talk about supporting the driveshaft..... I don't know how you plan to change the seal with it installed. Needs to be out, so you can get the coupler off the trans input shaft to put in a seal.
