Webster mk4-6 gearbox questions

[tumba]

Moved questions to Vintage & Historic General Discussion insted where it probably is more suited.

Hi!
I recently got what I believe is 4 speed webster mk4 gear cluster.
Before I put it in my car/gearbox, I'd like to convert it into a 5 speed.

While looking into this I noticed that all four gears were 9/16” thick.
When I remove the 1st gear blank spacer and reposition my 1st and 2nd gear on the pinion shaft, that gear pair seems to be 0,125” too wide (2x0,0625”).

So the question is: Do I need to change to 1/2” wide mk9 gears on 1st-2nd or all gears in order to have a working 5 speed gearbox, or is there a different 1st/reverse hub that I should buy?

Are the 9/16” wide gears of an older design that only can handle < 180hp?

Thanks in advance!