60 to 63 cm??? You must be TALL!
Can't help with mounting. Most folks with child seats who come in the shop have a compact frame or "women's" frame, either sloping top tube or step thru.
Step ladder?
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The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.