In order to target that player afterwards, you must use CommandStats to track the success of a command.

Prerequisites:

Objective to hold the return value.

/scoreboard objectives add AboveChest dummy

Applying the "AffectedBlocks" trigger to all players, who will then set their own "AboveChest" score depending on the success of block-related commands. This may need to run on a clock if new players can join at any time.

/stats entity @a set AffectedBlocks @a[c=1] AboveChest

In order for CommandStats to modify a target's score, that target must be tracked prior. This may also need to run on a clock.

/scoreboard players add @a AboveChest 0

Clock commands:

The following must be run on a clock in numerical order.

1. Cause players to run a /testforblock to find the chest. If the command is successful, that player will have their "AboveChest" score set to 1. If unsuccessful, that player will have their score set to 0.

   /execute @a ~ ~ ~ testforblock ~ ~-1 ~ minecraft:chest -1 {Items:[{id:"minecraft:stone",Count:64b,Slot:0b}]}

2. Target players based on their "AboveChest" score.

   /say @a[score_AboveChest_min=1] is above a chest containing a stack of 64 stone.