78 Corvette brake conundrum...

If the pushrod is too long, the C-ports are closed, and it will not gravity bleed. It cannot bleed at all because the ports are closed!
On another note;
If the master cylinder is higher at the front than at the back, it can trap an air bubble in the high point, that is gonna act like an airspring, When you push the pedal down the fluid will go down the line, and then when you let off the pedal, the air bubble will push the fluid back up into the reservoir. If you pump the pedal rapidly allowing it all the way back on each stroke,then hold it for a half a minute; then you can beat the bubble at his game and drive the fluid past the bubble. You can repeat that until you get a spongy pedal. Then you go to the calipers and force the pistons back into the bores, both of them, and make sure you clamp the offside one down. This will drive the bubble (hopefully) back into the reservoir.
Alternatively, you can have a helper crack the line as you are forcing the pistons back and drive the bubble to atmosphere.

You can know that you have a bubble, by the size and power of the fountain that spouts from the reservoir when you gently leave off the pedal after having pumped it several short pumps and hold on the last one;
No fountain probably indicates a closed or severely restricted C-port.
A flipping huge fountain spewing fluid 4 ft high,all over the underhood area, indicates the rear shoes need adjusting,lol, or shazzam that's a big bubble.
On a properly bled and working system, the fluid will just roil around in the reservoir with perhaps a small fountain no more than .75 inch high.
As you know,brake fluid eats paint, so take appropriate precautions