I applaud your friend's decision to alter the core support to accomodate a larger radiator. So many fellows are looking for the "magic bullet" radiator that will cool their monstosity without doing any work to the core support or changing it out to another support to achieve the goal.
I've done this sort of work many times in the past with engine swaps and have found that the cheapest and most bulletproof way is to use the largest radiator I can fit in there from the git-go, along with the shroud that was designed for the radiator from the factory and the designated steel fan and viscous drive assembly for same.
The one I like best is the radiator and support from a '76 Cadillac Fleetwood or Eldorado. Any of them from '70 to '76 with a 472 or 500 will work, but the '76's used the 500 inch motor for sure. Call around and find a boneyard that still has the fan, shroud and core support. You'll be using a new radiator and viscous drive fan clutch to bulletproof your installation. Make yourself a memo of the exact year and model the pieces came from so you can match up the parts.
You may or may not have to alter the fan clutch hub where it bolts to the water pump/pulley. Usually, the holes are slotted so you can make it work. If not, some minor surgery on the hub will do the trick. With the motor in the vehicle and finalized for position, bolt the fan clutch and fan to the water pump. Mount the Cadillac radiator and shroud to the Cadillac core support.
The Cadillac core support will probably be longer than the stock one in the pickup. What I usually do is retain the outer pieces of the truck support where it bolts into the body and cut the middle part of the truck support out with a Sawzall, leaving a few inches on each side. Then I measure the opening between the two stubs and cut the Cadillac support to fit into this opening. Remember, measure twice and cut once. It's better to leave a little more sheet metal on the Cadillac support until you determine the correct position of the fan where it engages the shroud opening. Then I position the Cadillac support with radiator and shroud attached up to the fan, equalizing the distance between the fan blade tips and the inner circumference of the shroud all around. Move the shroud around the fan until you have the fan blades halfway in and halfway out of the shroud opening. Normally, you'll have to tilt the top of the radiator/shroud back a little at the top to match the fan angle. If you need a little more front to rear clearance for mounting the support, you can position the fan blades 2/3 in and 1/3 out of the shroud opening. A little further in is ok, a little further out is not ok.
With that accomplished, simply attach the middle piece of the Cadillac support to the stubs of the pickup support. Use whatever pieces of sheet metal or whatever that you have to in order to make the connection. The Cadillac support may end up sitting forward of the stubs or a little behind it. Whatever. Just use your head and figure out how to connect the sheet metal, then MIG it in place.
Now, you will have a radiator that will cool anything and you still have the stock attachment of the pickup stubs to the truck so you can use simple hand tools to disassemble the whole mess later if you have to.
Here's a link to Radiator Express showing the dimensions of the Cadillac radiator.........