1275 Midgets (all? I think) had small journal rods, dimensionally the same as Cooper S ones.
The 12G1505 crank is a big journal one, 1.75" crankpins. Contrary to early popular belief, these are NOT `stronger' (ie resistant to cracking) than the earlier, small journal cranks. Crack test it first, before you do anything. Many here in Oz have cracks in, good only for doorstops.
A well known race engine builder here in Sydney always grinds them down to 1.625". His theory is they need some flex, or they will crack. IMO he may well be right...
As I said, if you don't grind the crank down, you will need big journal rods of some sort. I would use 1275A+ ones on a road engine.
1360 power- Morris 1300 auto block, S crank & rods, Russell Engineering RE282 sprint cam, over 125HP at crank, 86.6HP at the wheels @7000+.