If you had a complete donor - use the exhaust out of it - take a bit of fab but doable.. the 250 rad is fine for a stock 4200 ..might have to get creative with the hoses but otherwise good to go. The 4200 does not hod much coolant and does not need a massive rad to cool it.

51 GMC 4.2 turbo
Can't solved today's problems using the same technology/thinking that created them