Stone Block on ArmorStand:
/summon armor_stand ~ ~ ~ {ArmorItems:[{},{},{},{id:minecraft:stone,Count:1b}]}
or
/item replace entity @e[type=armor_stand,limit=1] armor.head with stone 1
You said you use an Texture Pack for Custom Models, if you're using CustomModelData in your Pack then this is your Command:
/summon armor_stand ~ ~ ~ {ArmorItems:[{},{},{},{id:minecraft:stone,Count:1b,tag:{CustomModelData:1}}]}