Depends a lot on the tonnes of the splitter and how many knives etc. Mine is rated at 60 tonne, 8 way, I used a 6inch x 3/4 top plate and the lips that hook under were 1x2 and have 1/4 inch of 'grab' but the beam was modified so where the 1x2 stopped, the beam started, to reduce sideways movement.
Give it too much grab and it has too much leverage on your beam. Sled was 1/2x 16 and we bent that (half the bolts on each side sheared off/stripped nuts off threads.) due to push plate being too high, operated upside down and only 4 bolts either side. Solution was to straighten, replace bolts, then run weld along the joints, and operate right side up.
The weld can easily be cut with a grinder and thin cutting wheel for servicing, and saves a shedload of drilling extra holes thru all those hunks of steel.
Good luck!