Going from 4speed to 727 in 65 dart question

What motor ?
I ask because my '65 had a 727 in it and the installer ( previous owner ) used a small brain and a large hammer.
I had to weld patches in the floor where the seam used to be.
You will need to clearance the tunnel at the seam and there are a couple ways to go about it.
I might consider removing the seam and welding sheet metal on the top side to close it up.
A 904 is a much better option if you are going small block.
They will hold up and don't have the forward drum issues like a 727.
Do some homework on the 904 and I think you will agree with me.
I have a few bucks into mine , but it will hold up to everything I plan on throwing at it ........ a 405 inch R-3 / W-9 motor that will be well north of 600 h.p. on E-85.