、。
If I were you, I'd propose to her.
Literal
I [subject-が] you [if-なら], she [to-に] propose place is [but-けれど].
~ところだ after a dict-form verb expresses 'would be about to [verb]' or 'would [verb] in that situation' — a hypothetical conditional consequence. Combined with 僕が君なら ('if I were you'), it becomes 'I would propose [if I were in your situation].' The trailing けれど ('but, though') leaves the advice as a suggestion, not a demand.