Dave, alittle over a year ago did the opposite of what you're attempting in a 49 GMC. The GMC is similar to the Chevy except the GMC's had longer engines by a couple inches (like the more modern 230/250/292) so GMC engineered the radiator further towards the front for clearance.In this video
you can see how much room a 79 250 takes up in a 49 GMC 1/2 ton. If you look at how the radiator is mounted you'll see the difference between how GMC mounts the radiator further forward compared to the Chevy radiator mounting. The engine is too low and too close in the GMC for either a clutch fan of flex type fan and a std fan would have hit the bottom front cross member. The previous owner had an electric puller fan that just fit.
These pics show how the 250 is mounted in the GMC frame.Pic 1Pic 2
The mounts are stock Chevy clam shell type and the bottom mounts that were bolted to the welded on perches also lined up perfectly for the 350 engine I swapped in.
The difference was the upper mounts that bolted to the engine block.
If I read you post right it appears you already have the mounts installed in the frame for a SBC. If so you should be able to bolt a 230 or 250 to those lower mounts with the correct L6 upper mounts. A 292 uses and offset lower mount.
You'll just have to resolve the waterpump to radiator clearance which would seem to entail mounting the radiator alittle further forward.