What freewheel/cassette are you riding? We all raced and trained on 52 (53) - 42 back in the day and used small closely spaced freewheels. (Rarely more than 24 teeth. But - we never tracked heart rate. (Oh, sometimes I'd put my hand over my heart at the top of hills and multiply 6 seconds of beats by 10. I only did that when going very hard and tried to see if I could get over 200 - simply as incentive to go really hard, not as a training monitor.)
The bikes and gearing of the old days don't work with the modern constant RPM and HR thinking. The 52-42s and narrow FWs much more so than the earlier fix gears but still basically incompatible. Getting over the routine hills on the famous "Allis Loop" of Boston with a constant HR on the bikes John Allis rode? If you'd suggested that to him, he'd have laughed in your face.
Now, I spent nearly all of my time outside of races, downhills and speedwork on the small chainring. Hundreds of hours every summer on the 42-14,15 and 17. Hill? The 19 and stand. Rainy day? The fix gear with its 17 or late summer, 16.