This is dangerous stuff but...
You can rewrite the whole partition table ( removing then adding back in the partitions ). But this can go wrong ( So don't blame me! ). Theres another option which is rescue ( See
parted manual ) Which may be better.
Resizes are never the best bet. When you accepted extended it seems to have wiped the partition ( Logical was correct ). Logical partitions have space available either side of them, also the have to end or start on a cylinder, which may have been why parted wouldn't accept the new partition as it was.
To achieve what you want, I'd of backuped 5 and 6 removed the 5, 6, 7 partitions. and added the new ones back in. You can try to set what you want now though 6 ( now 7 ) will be the hardest to set right.
8 wouldn't be the new partition, it would be 6, ( 6 and 7 would of been increased to 7 and 8 ).