Hello redcar98,
What model Del Morino Flail Mower do you have?
You may have a great deal of fine dust in the adjuster track that is preventing the
adjuster from fully releasing to allow you to install the V belts if they are the correct ones.
If you have fully released the snubber pulley and you cannot slip the V belt(s) on
the small pulley and then the larger pulley you may have been sold the wrong V-belts.
You only need hand tools to release the tensioner/v-belt snubber for
the V-belts otherwise you will ruin the new belts.
Loosen the V belt tensioner completely and then place the V belts on
the pulleys and then adjust them.
Do you have a V belt tensioning gauge??? If not, you should have one in your tool box.