The 258 radiator is a different size from the V8 radiators, and may be hard to find. The core support only fits the six cylinder size radiator.
When I repaired the radiator in my J10, I took it to an actual radiator shop and they replaced the core. There is a company in Maine that makes the cores, and the box with the core was labeled "Utility Truck" so the six cylinder J-trucks appear to have the same size radiator as the Willys Utility Trucks.
This was maybe a decade ago, and it was ca $300 IIRC. I'm sure it will be quite a bit more now. Most "radiator shops" do not actually work on radiators today, only R&R commodity radiators. You will need to search for a shop. Ask your local car clubs if you can't find a shop in your area.
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk