Is 1/2"x28 an acceptable substitute for 13x.75? Would this make the barrel wall too thin for a .270 caliber? The overall barrel diameter at the muzzle is .8"

.800 barrel dia. you can do a 5/8 ×24 which is exactly what I would do

1/2"= 12.7mm so the 13mm would be very close as far as diameter goes