Don't know if the install height is different between mag and LA head.......
Either way, a Chiv(Chevy) valve is similer in stem diameter as the mag valves and cheaper.
Would require a machine shop, either way, as the valve stem diameter is : LA 3/8"diameter, mag 8mm and chiv 5/16(?)
all will require the guides to make a different size.
stem install height cam be adj properly while the head is in the shop.
I have a brand new set of Mopar valves with the 3/8 stem and magnum heads that I bought for my 340, but went with magnum heads instead. I'll give them to you at a good price. Shoot me an e-mail.