If pastrami is cold, wrap in 3 layers of plastic wrap and warm for an hour in simmering water (not boiling).
Cut 4 slices of rye bread into chunks and pulse in food processor until broken down to coarse breadcrumbs (no large pieces remaining, but not too fine). Toss in bowl with melted butter and caraway. Place on baking sheet in 325 degree oven for 15 minutes until golden brown. Remove and set aside.
To build nachos: Cut hot pastrami in 1/2 inch chunks and toss in bowl with deli mustard.
Arrange layer of TOSTITOS® on platter, top with half of the pastrami, and sprinkle with half of the cheese. Repeat with another layer.
Place in 400 degree oven for 3-4 minutes until cheese is melted.
Scatter pickle slices over top, sprinkle with bread crumbs, and serve.