The stock motor mounts for the GM K member should be bolt-on. They should have two different mounting locations for SBC and BBC. All of the A,F,G body cars came with the cross member drilled this way for the 3-bolt type mounts.
The landing dimension changes for an SBC and the BBC. See the photo below; these mounts are set in the BBC holes, the lower holes are for SBC. The holes also have a 3-nut cage inside the K member that holds the backnuts in place. Your welded on mounts appear to be set for a BBC, they are pretty wide.
Many times people will weld-in the mounts if thats where they are keeping their motor.... but we all know things change! Also if you are setting up a BBC in a F,G,A body frame you need to offset it to make it fit right. Keep that in mind.
I would see if you can find these stock GM mounting holes again and use them for proper direction first. If not get some measurements from somebody with the same K member as yours.
As for your radiator... it looks like somebody setup a 'saddle mount' type radiator in there before. But, if you place the assembled front clip onto your frame and locate your "radiator support" mounting holes that way, you can use the original style downflow radiator intended for the truck. Just make some "L" brackets to weld onto the frame wherever they end up and weld them onto the frame where the radiator support lands.
As long as your 'support' is located in the right location for all your body parts to mount to it, then everything should line up in the end.