Small block 426 stroker with trickflow heads. How much HP.

Too small. 1 5/8" step to 1 3/4" TTI's into 3" exhaust with an X pipe and Dynomax mufflers.

they may not hurt you as much as you think.
I ran those headers on a pretty mild 416 years ago
3350 pounds with me in it..
416, excellerator with no work, mildly ported eddies(275 max)
260/266 600 lift flat tappet PRH specced, ran low 10.70’s at 124
11.5 compression. 727 tranny