both are stock in 77 . the 15x8 snowflake didnt come out till 78 with the WS6 , but guys use them.
but if I had to choose , I like the rally over the 15x7 snowflake.
the widest tire I would go with on a 7" rim would be a 245-60-15 some go 255-60 on a 7" rim . but I think its to fat.
my 79 the 8" flakes with 255-60r on all 4.
and the 77 has the stock 225-70 on the 7" snowflakes