How much horse power will I gain by switching out the heat exchangers from a 1975 to the pre 75 dual exit heat exchangers and adding a Monza muffler?

3 - 5 maybe.

Horsepower is not the only consideration though.

I backdated the exhaust on my 76 from completely stock to 74 style SS heat exchangers and a Bursch quiet extractor muffler. I liked the result but I can't really say I felt a big increase in power. It may have just been the exhaust note but it felt as if the engine was a bit more lively above 3500 rpm.

I will say that many years ago I tried a Monza 4-tip muffler on my stock 76 heat exchangers. I found it to be too noisy for my taste. At first it was fun but after a couple of months I found I was driving to reduce the noise. I would coast into my driveway because I didn't want to annoy the neighbors. I know, what a wimp. But in less than a year I had replaced the monza with the stock muffler system (had to buy a new secondary muffler which are now NLA I think) and thought good riddance. Several years later when I went to the Bursch quiet I was a little wary, but I found the sound to be reasonable. IMHO, if you go with a Monza just be prepared for some noise.

I will say that many years ago I tried a Monza 4-tip muffler on my stock 76 heat exchangers. I found it to be too noisy for my taste. At first it was fun but after a couple of months I found I was driving to reduce the noise. I would coast into my driveway because I didn't want to annoy the neighbors. I know, what a wimp. But in less than a year I had replaced the monza with the stock muffler system (had to buy a new secondary muffler which are now NLA I think) and thought good riddance. IMHO, if you go with a Monza just be prepared for some noise.
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everytime I drove through Apt complexes........I would set
off all the car alarms.....I got tired of that!!!(went back to stock)

I actually just did a 2.0 backdate to my 1975 1.8L. It seems much more lively, and a step or two quicker according to my butt-dyno. Then again, I also had the valves, dwell and timing adjusted at the same time, so it could be a combination of several things. I did notice that it ran much cooler than the stock setup.

Thanks again to everyone at the Sacramento swap meet for setting me up with parts and advice!