Ouch! Nasty deal, this one. I'd be sad.
The backpressure difference between them could be made up by baffling the HM300 pipes some more, though, if no other avenue raises its head.
Here's the "inside" story: look at the back side of the HM341 pipes, watching to see the little spot weld dimples that hold the internal baffles in place. Most of them have 5 of these, about 4-5" apart. There are some that have 3 instead, with another set (seemingly #4) of spotwelds near the rear, about 1.5" from the last real baffle: this is the little cage that holds the last (exit) black plug in place, and doesn't really count as a baffle. So, you should find 3, 4 or 5 baffles inside.
Each baffle has a 12mm hole in it. They act like a choke to their chamber, or a restriction of 12mm from the end of the header pipe to the end of the muffler. The last hole is a little larger and longer than the front ones. This last one is responsible for 2 things: it breaks up the throaty rumble of the last chamber and also forces the exhaust to hit both the upper and lower pipe's chambers so as to spread out their resonance and expansion: this actually causes a slight header effect at engine speeds above 6000 RPM through the little rubber hose connection between the pipes.
A simple approach might be: if your HM300 insert can be removed, you can then mimic the multiple 12mm restriction holes by using the baffle (or one of your own fabrication) by wrapping over the fiberglass batting by either sliding a steel sleeve over their whole length (snugly, please) or by wrapping it with some thin sheet metal, maybe like gutter/roof flashing material. In the length of the tube, you would have to insert as many 12mm washers as you have baffles in the other pipes. Install one right at the inner end of this baffle, another about 1/4 of the way out from that one, and split the difference for the remaining one (or two) to the outer end of the baffle, which is now a pass-through type. The chambers are going to be smaller than the HM341 chambers, so if there were, say, 5 baffles in the HM341, I would be tempted to use just 4 here, because the pressures will be higher overall. Thus the backpressure will be similar.
The only real drawback is: the efficiency of the new baffle will be lower than the HM341 versions. This will cause some power loss to the HM300 side at higher RPM. You might find that the 2 HM300 cylinder still run leaner than the other 2, though.
The opposite approach: you could 'loosen up' the HM341 pipes a little bit while also slightly increasing the HM300 restriction. This approach would bring a closer balance overall. To do this, remove the end plug on the HM341 pipes and drill 3 or 4 new 1/2" holes through the last baffle at the end. You'll need one of those real long drill bits or a bit extension to reach that far, it's about 5" down inside on the ones I have. Then reinstall the plug, and be sure to connect the little rubber hose between the pipes. Then, on the 2 HM300 pipes, plug the hose holes on each pipe, or put a plug into the rubber hose and reinstall it (either way works). I once made a metal disc from a knockout of an electrical outlet box after I installed some outlets in my house: trimming it down to fit inside the hose and then capturing it between the 2 pipes worked great. Then, pull out the HM300 baffles and study their outer end: if yours are the type that have just a 14mm hole out the final end of the baffle, you're all set. If instead it is the kind that had the center hole AND the open ring around that to let out more air around the baffle pipe, you'll have to close that part off.
This second approach will produce a nice (and unique!) combination sound of both the crackly fiberglass baffles and the throaty pop of the HM341, together. Yours will be a very distinct tone among 750s, which will stand out if you ride with others. At worst, the mainjet of the 2 cylinders with the HM300 pipes might need to me 2.5 to 5 larger than the other cylinders, but make sure all the carb needles are in the same slots. I would recommend using the 4th notch (that's the 4th clip groove from the top) and a #105 mainjet to start out. If the 3-4 plugs constantly show up whiter than the 1-2 plugs under your normal riding, then increase those 2 mainjets to 108 (the 107.5 size is usually marked as 108). Make sure the carb float levels are all the same: I'd recommend using 25mm instead of 26mm levels here, as the natural air pressures at higher engine speeds will tend to settle the bowls to a similar feed rate level (not the same as an actual level). If you have my book, look in the Carburetors chapter for the picture where I show the 2 vent tubes entering the airbox, where I drilled 2 1/4" holes to accept them. This will create a better bowl pressure regulator that corresponds to the actual engine speed, which is something done on later bikes by many manufacturers. It tends to help the fuel drain faster into the bowls by making the bowl pressures steadier and more equal left-to-right.
