Pacesetter in recent years changed the G180Z/G200Z header port from square to round(my old style has square ports), it is now essentially the same header as sold for the 2.6 4ZE1 engine. Very early models also had a 3" collector rather than the current 2.5". According to someone who bought one of the round port models and installed it the round header ports are large enough that they don't overhang the head port except slightly in each corner(this is also Pacesetter's claim). However most Pacesetter headers don't match the head ports all than well, I suspect these are the same. I would do some minor port matching, there is not enough material to completely blend the round to the square. You can't use the exhaust gaskets as a template either, they are way too big. This is what I did on a 4ZD1 head/header: make a separate template of the head ports and of the header ports. I used thin cardboard like a cereal box, aligned carefully using the mounting holes, then tapped around the perimeter of each port with a plastic hammer enough to imprint the port onto the cardboard, then carefully cut out each port opening with an Xacto knife. Then carefully aligning the head template on the header and the header template on the head mark any meta that protrudes into the template opening and grind out the marks, chamfering back fairly quickly(you can grind all the way to the tube side of the header flange, the tubes are welded to the flange away from the opening). This will smooth up the flow through the head to header transition, whether it is worth the effort I can't say. The exhaust gasket is large enough that it sits back away from the opening. The Gaskets Pacesetter provides are poor quality, I suggest replacing with something like the metal clad FelPro, however before tossing the fiber gasket test fit the header, on two occasions I have found the clutch arm contacts one tube, requiring dimpling the tube a bit. Doubling the fiber gasket with the FelPro gasket adds just a bit more clearance. At the collector the Pacesetter gaskets also fail quickly, I use Mr. Gasket #5980, usually two stacked & sealed together with hi-temp RTV. your collector is 2.5" so that's the max tailpipe size, I think the reducer supplied with the header is a 2", which is probably about the right size. Too large of an exhaust pipe will lose some low end torque, plus it will be very hard to muffle(not enough backpressure to combine the exhaust pulses can cause a "chuckle"). Err on the large side, you can always add a restrictor reducer on the tailpipe end if it's too big, if it's too small you replace the pipe. You can also install a Delta 260 camshaft for more low end & a Weber carb doesn't require sufficient rpm to build enough vacuum to open the secondaries, so more low end throttle response. IMO re-configuring the header design won't be worth the effort, I would at least try it first, then at least you'll have a base for comparison should you decide to go ahead.
_________________ '78 Chevy LUV,1.9,4sp,headers,31's,SAS Dana 30,Dana 20,SOA rear w/Aussie Locker, rough body, bought new 12/4/78. '87 Trooper,2.3,5sp,headers,31's,ball joint flip & spacer,Aussie Locker rear,Superwinch hubs,brush guard w/5.5K winch,more to come.
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