The E15 dizzy swap is so easy even woody could do it.
If your springs are old, they will start bouncin sooner. I ran competition springs, (doubles) part noCB4A02 in my blown A14. Hit a gear too hard, and the tach used to flick around to 8K instantly.

Thrashed old A12s Ive had in the past have always been screwed out to 6K. Given enough road, they could usually do it in 4th too.
Increase to breathing, give 'em more valve spring, and some more cam. Then Bob's ya uncle!