Go easy w/ that 1/4" tap, by the way. When I did mine, the set screws afterwards were a little too loose for comfort on the first few I tapped. I later used a tapered tap and went a little less deep, leaving more for the set screws to bite. That was just me, though, your experience may vary. It could have just as well been that particular set screw's fault.
So, yes, you can use you present heads with the replacement block. Check out Dan's Auto Service, I picked up a complete 360 (which needed a full rebuild) for $150 1 1/2 yrs ago. He practicaly gave me a running 727 last fall, cause it was left out without a dipstick tube, getting water in the pan. Hey it's AZ, just how much water ya think got in there? Yeah, that's right......zero.