Try to contact me in private and give me access to your problem to see why is taking so long. Do you have many templates? Do you have many posts per page? What is this custom field you’re trying to access?
Regarding editing the loop template if it helps in the main section set Content Width:Boxed with 300 value and set preview (click eyeball > settings) as a post in that archive so in this way you’ll see roughly the way would look inside a post grid.