Hey guys, need advice...

Im building a a12 with a14 crank and a15head.
Will a lightend Connecting rod help?
Or would it just weeken the part?
I will be using a a12a conrod..

All I've ever done to the conrods was shotpeening, balancing & one time polishing. Show your standard datsun rods to a Chevy or Ford V8 mechanic & they'll weep. The Standard rods are tough enough with a little work for what your doing, unless you do like an off road racer did once & number them with a file. Instant stress risers. You could use ARP con rod bolts if you want, but you can get by with the standard ones.

If you look in the wiki Dodgeman with the help of DDgonzales got some a12a rods which have larger pins to suit the larger pin A15 pistons. Instead of opening up the early rods (that is if you have an earlier engine you are building) to suit.
At this stage you save money on cheap rods and make up with a lighter flywheel, A14 pulley (lighter than A12), maybe a little lightening the crank and the rest your call for a revvy high torque hybrid.

A14 crank in A12 block doesn't need any "help". It will rev to 8500 RPM with stock pistons and rods. The A12A rods as mentioned have a different size pin, so the A14 pistons can be directly fitted. Otherwise one can hone out the A12 rods to fit the larger piston pin.

Shotpeening and polishing add even more longevity to the rods.

Btw, A15s used cylinder heads designed for A14 engines.