All the joints were drawbored, and I made a bunch of dowels for the project. I can't quite remember what size, but I think they were sort of 3/8" in diameter.
I used my homemade drawbore pins to test the fit of each of the joints, and they worked really well. Having made four of those enabled me to test both ends of a stretcher/leg assembly at the same time.
The long front stretcher was made with a triangular shape at the top, to accommodate a sliding deadman. I even remembered to make a slot in the front legs for the parallel guide for the leg vises before assembling all the parts!
I placed the top upside down on a couple of battens so I wouldn't mar the top in case there was a small stone or any other debris on the workshop floor.
The tenons and the mortises were lubricated with an old candle, as I didn't want things to seize up half way. The undercarriage was brought up to the mortises, and I double checked that the front of the bench top was also aligned with the front of the undercarriage before I began to negotiate the legs into place with the help of a hammer and a block of wood.
Slowly but surely the legs seated themselves in the mortises, and once the sound changed upon hitting a leg, I knew that they were in position.
The assembled undercarriage.
Checking that all parts line up.
Checking both sides.
Half way through.
About an inch to go.