If you absolutely have to, you could notch the frame a little. But I would be careful to not get into the VIN on the frame right there. Considering the fact that you have a frame clearence issue as well as with the alternator, I would look for a different bracket. I don't know what the bracket you got from Summit looks like, but it obviously isn't going to work very well for you. Have you looked around for a pump bracket for the '67-'72 trucks. Although I can't say for certain if they will fit that pump. I'll look around a little and get back to you.
Edit: OK here's one on Ebay.....this one should work for you. Since yours is a 4x4, you might have to modify the bracket slightly, dunno for sure.
http://cgi.ebay.com/ebaymotors/67-72...Q5fAccessories
Here's another one....looks good.
http://cgi.ebay.com/ebaymotors/67-72...Q5fAccessories
Here are a set of brackets only. The one bracket in this could be modified some if needed.
http://cgi.ebay.com/ebaymotors/1968-...Q5fAccessories
There are some others, mainly repop looking brackets. Just do a search for 'Chevy truck power steering'