are heads worth the money?
It depends...
On a stock truck? bang for the buck, do the recipe type stuff... look on Sportmachines, read up on the board... I wouldn't go with heads until I redid the motor with strong parts and had money to really fund a good buildup.
how much hp increase will good set of l-35 heads give me?
It depends...
On what's in the rest of the combination. If you use a cam, a larger turbo, good fueling & management, and go from stock heads to ported L35s... you'll see huge gains. If you are on a stock truck and go to L35s... might not see much gain at all.
Yes, the stock heads don't flow much... but the stock cam is very weak, the stock exhaust manifolds start choking off then too, and the stock turbo has horrible back pressure...