Yep. If you've got a '76 prop valve bracket, it's the kind that mounts the valve to the top of the steering box, and if you've only got one, then you're only halfway there anyway, since there are actually two brackets. One bracket bolts to the prop valve, another bracket bolts to the steering box, and the two brackets bolt together.

If you're set on using the brake lines you have, you'll need to get the prop valve and mounting bracket that matches those lines.
Now...having said that....you can use a '77-1/2-'79 bracket to mount your '76 prop valve on the back of the front crossmember, but the lines will require some minor tweaking (since the later prop valve is a little smaller). Here's a picture from a member here where he did just that:

I was going to post a picture of the later prop valve, but I can't seem to find a good shot of one. But anyway, to do the job without having to do any modifications to the brake lines you want to use, I'd recommend just finding the matching prop valve and bracket.