Question about developing planes

This is a question for the devs or someone who has experience in this field:

Let’s say that you were to develop the Boeing 757-300 (Pleease). Would it take just as long to develop the model as the A350 or is it: take the model, tweak the settings, then lengthen it.

Given that therea aren’t many possible liveries, would development time be quicker and easier? Would it be viable to make a 757-300 or 737-600 for free to temporarily pacify the cheepos?

Thank you and…

Best, Boeing707