Friction shifting system as a bridging covers the variation
between various mix and match parts.

dropout spacing frames had to get wider as the gear count went up,
from 5,to 6 and then again to 8.
7 was a crowding in a cog on a 6 speed 126, spacing, 8 is adding a n+1 cog
at the 7 speed inter-cog spacing.

120>126>130.. MTB .. 135

Have 5 want 8?, shop for a new bike, it will be cheaper.