The rear axle is higher than he front. A 24 inch wheel (that will give you an effectively lower gear) is one inch lower than the 26 inch.
For the second bike I will be using a fixture for the bottom brackets that will help keep the two halves of the frame straight, when clamped together for tack welding.
These hold devices should be mounted on a metal flat bar at least 5-1/2 inches wide x ¼ to 1/5 inch thick. Do not trust wood to be square and straight.
A frame jig of 1/2 inch steel plate would be very nice to have.