Pete, to ease your pain, it has happened to me too!!
Uni student, wants 308 stroker for panelvan, saves, saves, buys block, buys second block, machines second block, small crack, sh#$, "no worries" says machinist, "ill just plug it!!" Ok, block done, I assemble motor. Botom end done, cam in, home ported heads done, painted, trick manifold and carb on, pull front end of said panno, and in goes engine!! Yay!!! Fire it up Double Yay! Sounding fat out of the pipes, 20 mins to break in cam, all ok, stop check oil, Ohhh NOOOOO milky oil, manifold off, compression test on cooling, OOOOHHHH NOOOOO leaks!!! Heads off, more compression test.......DIE!!
The plug that the machinist had installed on the block and assured me was ok had pushed one sleave away from the water passage, and water was POURING into the barrel, even with the low pressure of a bike pump on the radiator filler!! Soo much time, money and effort down the drain!! sooo start again, and change machinists, second build and 4th block later, success!! You will get there in the end, and good luck with the new moter, your GT40 is looking fantastic!