You can rig up the radiator and pressure test your work.
This is a Jeep 4.0 being tested. Spray each core plug with soapy water, radiator tester pumped up to 15 - 18 psi.
Of all the things to buy from a junk yard, i don't think if would really be worth the effort to buy used body mounts, new ones should not be too expensive.
I have used these as replacements for 500 Watt lights. I remove the old housing and wire in these new lights.
https://www.amazon.com/gp/product/B07X42GGV8/?tag=atomicindus08-20
Kill Trap
https://www.amazon.com/gp/product/B00K6JKVK4/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
I had a squirrel problem, 40 kills since April 2020