Ahh, you only have one of these...

This is where you get into the situation I'm in. If you design your machine around scrounged/repurposed parts, you get locked in to some decisions that, without that scrounged part, you might make differently. Ask me how I know!

I'd seriously think about just using this part as a pattern and if you're prepared to make a new part to match this, why not go all the way and make a gantry end that can take two blocks? While you're at it, make two...

Anyway, just a suggestion.