There should be a tab welded to the tube, a few inches from the top, that is bolted to the rear-most stud on the front cylinder exhaust manifold. Once the tab is free, there is about an inch of tube that is pushed into a hole in the block.
I found it easier to deal with when doing the heads, but that's hardly a reasonable solution if all you're after is the tube - not a lot of room to work otherwise...that tab is on the same stud as one of the heater tube tabs, as shown in the included image. The weld for the tab to the tube can be fragile, so be careful. Also, be sure the tab alignment against the manifold/on the stud is correct before you sink the lower end of the tube into the block. You don't want to be twisting/bending that bracket on the tube to get it to sit properly.
I use a blunt punch to gently drive the tube into the block, tapping on the either rib, with black RTV to seal it up. Coat the outside of the end of the tube, then put it in the hole and leave it alone.
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