Changing from a small block to a big block in a 68 dart. This car originally had a big block in it. I still have the small block motor plate. Is it possible to just lay the old small block plate on the new big block plate and transfer the mount holes to it? I guess you would need the measurement difference between the two crank centerlines to make the big block plate the same, but I don't have any small blocks around to compare it with right now.....

Is there an easy way to do this without making cardboard templates and trial and error?

From the center of the crankshaft to the top of the K-frame the correct distance is 5-1/4".
The engine is also offset toward the passenger-side. Measure from the center of the crankshaft to each frame rail. The
difference should be 2-1/2".

That should do it for ya

You might want to set the BB in there with the headers
and center it between the rails for more room.
Also if the SB was set up higher than stock(vertically)
you might want to lower the BB for the added torque
unless you had traction issues before. The higher you
set the engine the easier it is to get the weight transfer