Just me but it's better to have the lifter full of oil and the plunger at the top of it's travel when adjusting the lash. Do it with the lifter plunger bottomed out and once the lifter pumps up, then you've got the valves hanging open.
Yea, that's the way I do em. With my Canfields on the 331 , Z cam adn hyr rollers, I adjust the slack out of the pushrod and tighten another 1/2 to 3/4 turn. Works fine.
If the lifter plunger is at the top of it's travel when you adjust them, they won't. You may have bought into a set of instructions I too have read somewhere. It said to bleed the lifters all the way down then adjust the lash. If you do that, the valves will hang open when the lifter pumps up.
I'm chiming in kind of late on the 289 head options but when I bought my car it had a 289 w/ 351 windsor heads. I believe the heads have seen additional work but even stock aren't the valves larger than stock 289. I'm certain that I'm pushing 300+hp. When I had the engine rebuilt my builder thought 350 but I'm guessing less. Anyway, seems like a possible option to increase flow without going to aftermarket alum heads. I imagine you could get a pair fairly cheap.