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The 450 ohm characteristic of the ladder line has no bearing on its use for a J pole since except for the gamma match at the bottom only a single wire is used above that.
I built a couple of J poles from twin lead and the results were disappointing. It was very difficult to get a decent match and it was a critical adjustment requiring adjusting both the length and the feed tap point. Add to this the complication of a 5/8 wavelength which gives a high feed point impedance it may not be worth your effort. Even when I achieved a match it didn’t radiate as well as a simple 1/4 wave over radials. I threw it away.
No harm in trying and if you get it to work let us know how it went.