If you stand the gearbox on its rear flange, then lower in the converter in from the top, even though there is an alignment tool for the converter splines it can normally be manoeuvred into place without it, (and if that wasn't possible you wouldn't be able to do it the way you propose without the tool anyway), then fit the box, bellhousing and coverter to the engine as a unit. All on the level.
A 3 litre engine, having a DG box lowered on to it, on an engine stand, the way you propose doing it, my only advice would be don't buy the engine stand off ebay.....
I bought my engine stand off eBay several years ago, and it supported the 3-litre engine attached to the bell housing mounts. It has stood that way for most of the last year.
I have had the torque converter off before with the engine still in the car, and have reattached both without the special tool. Engaging the gearbox is possible, it was a question of aligning splines first, and advancing while gently rotating the output shaft until everything clicked into place and slid in.
But it is not possible to easily check the torque converter for run-out with the engine horizontal, given the flexibility of the back plate.