Big block k-frame to small block?

Long story short, bough this 71 Duster in pieces.
Now i'm working to get that small block in.....after not understanding why everything was so off...i found out little late in the game that I actualy have a big block k-frame.....but i currently have a small block in the hole....my engine was sitting on wood 4x4...so only tonight i found out this.

So I would like to know if theres a swap big block to small block conversion....later maybe i'll put a big block....but for now i don't have the money or place to swap in a big block.

Any help appreciated.