if these stock gt40p vs Thumper are the only option you are considering,
I would go Thumper, Thumper heads flow almost well as out of the box aluminum heads. i have seen it on a bench! and his porting price is very good.
for most competition porting/valve job, it will cost right around $1000 or more, for what he does and his workmanship, his price are very good.
Personally i never bought anything from thumper, i have considered it but i have seen his heads at local porting service getting new springs put on and his heads flow! I was surprised, For 302 or 306 thumper heads are very ideal.
Good luck