I think the 35mm box gives perhaps about a stop more light than the 120 and maybe 2 stops more than 5x4. Have never measured the difference but if you have the 35mm box you'll get far shorter exposures than 5x4. Ilford used to sell all the three mixing boxes plus the adapter plates for many different enlargers for £15 a set. Wish they still did.
Long live the 500MG and 504 combo!