early abody dougs d450 headers1 5/8" tubes .. 2.5 collector , Stock 65 dartcharger 273 .... Question is>>> what size headpipes off the headers would work the best with this engine? I have an old set up 2.5" headdpipes dynomax ultraflo muffs and 2.25 tails . do you think the 2.5 head pipes are tooo large ? or will work just fine .
This little engine does not have a lot of torque as you all know...... so I do not want to take all the bottom end away with big tube pipes.

I would stick with the stock single Commando exhaust from the headers back. You'll have to fab into the single exhaust. That's what I have with the exception I'm running a Flowmaster instead of the stock muffler.

2" would be fine, and no bigger than 2 1/4 if you ever plan to hop it up a little during a rebuild. Most people notch the passenger side of the torsion bar crossmember and transmission mount to keep the pipes tucked up neatly on both sides. Be sure to get the muffler shop that makes your new system to use aluminized pipe. See if they will let you brush the welds and treat them with Eastwoods manifold coating while it's all fresh and up on the rack.

2" would be ideal with a good flowing muffler.Consider a 2.5 mandrel bent single for ease of fitment and less weight.