No.
You are given a choice to BUY one or the other at current market values of a core, long or short block. Then and only then you subtract the cost of the engine block from the $3000.
What you have left over is what you can spend for ANY parts and machining.
No parts can be from your stash. No FREE parts from friends. Every part must be open sourced 'Fair Market' Price that everyone else gets.
Seem the average price of a 318 long block runs $150. Average price of a 340 long block runs around $1000. So the 318 now gets a $850 head start.