I was thinking that the original pedal return spring might be too strong for the new master cylinder. You might try without it or maybe shorten the extra link so it is looser when the pedal is almost retracted. Or your toeboard bumper idea might be good as well.

Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.