When I rebuilt my truck engine, I used a high powered steam cleaner, removed the freeze plugs and spent two hrs on that block. Then I tried to pull the tube, I tried all kinds of ways to no avail, so a perfect novice, I made a long slender spear and worked that all the way in, then I made a puller with a sharp strong end and found all the bottom openings, cleaned them and tried to pull it out. No success.
Then I looked down that tube with a very bright flashlight and saw all was nice and clean. So frustrated and tired I made certain the tubes' open end was like a funnel so the water had to flow through it, put the water pump back on and now have driven it for five years and it never runs hot, temp usually at 160.
The tube on my '53 suburban came out nice and easy and was in quite good shape, I put in a new tube anyway.