Change from 4-speed to 3w/OD

What it probably amounts to is a combination of factors
1) insufficient clutch departure
2) a tired brass or glazed cone
3) unfamiliarity with the 54% split
4) driving style
5) fear. Fear of taking charge of that shift.

Yes that split is pretty wide, and after you have driven it for a while you learn to blip the throttle, cuz usually the Rs fall faster than you are braking, and the brass cannot increase the engine speed,which may be dragging on the disc, so the operator has to bring it closer to the driveshaft speed, with a little blip, and then whamming the shifter into first. Exactly like driving a graintruck on soft ground; a blip and a wham. But the more familiar you are with this split, the easier it becomes.
The problem is you only have two feet, and one is holding the clutch pedal down. and the other is braking.I think this is partly why 340s had the dashpot on the carb, to slow the Rs down back to idle, so neophytes wouldn't wreck the clutch teeth during the warranty period,lol.
Anyway, what I often do is push the clutch in with power still applied. This causes a little rpm flare, and an easy synchronization. It's just a different driving style.
Also, if you're running lotsa idle timing, you're throttle opening is gonna be pretty small, and when you lift, the engine rpm is gonna tend to drop like a rock,which can make a great compression brake, but it can also make downshifting, a bit of a chore. That's another reason I only run 14*, and also why I kept the stock heavy flywheel.