Did you ever run the 383 with the exhaust removed? How bigs the carb and cam?

Even though you ran the same heads the pistons depths difference between the two motors could change the compression abit or a bunch.

Also like stated with the volume of air and fuel being pulled through with the extra cubes you could be running out of fuel.

Witch intakes are on each motor. A good tell tale sign of the second half loss of ET would be the MPH difference between the 1/8th and the 1/4. At the 12.5 zone it should be about 24 to 25 mph.

As simple as the change seems , theres still lots to consider.