SMALL BLOCK HEADS

Just curious of what you'd prefer. Speaking of stock 318 2bbl engine, per say for conversation, in a '87 Diplomat. Would you prefer a 4bbl installed replacing the 2bbl all else remains unchanged or headers with 2.5 duals replacing the single exhaust/cat system? IF you could only do one of the mods to the car to lower the ET max, which would it be?
headers and 2.5 exhaust would be best i m o m can i put in a bigger 2bbl at the same time ?