Mount them on the side (cap off the old holes), there's no point adding 90deg bends if you can get away without them!...
I'd use the original tube as well, it's got a much better inner diametre size than a 1/2inch barb!...
If you're sold on the idea of barbs though, I'd use a 3/8BSP brass nut to solder onto the rad, then use large 5/8barbs (14mm inner, you can get 14mm ones with a 11.5mm inner diam' as well), they're perfect for zero flow restriction with 13mm inner silicone/PVC tubing...
That's a nice size rad, I hope you're gonna make a neat slimline cowl/shroud for it?, to get tjhe most out of it
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