Trying to see what size push rods I need for my 408 nitrous motor with ls3 trick flow 255s with Morel 5315 lifters with stock ls3 rocker with t-upgrade....

 

You have to measure for length. If they will fit go with a 3/8 diameter pushrod all the way.

 

3/8 clear no problem but length has got to be measured to get it right.

 

Their web site says the guideplates are for 5/16" pushrods.

 

You don't use guide plates with stock rockers.

 

It takes measuring to do correctly. Basecircle, head gaskets, block milling etc all add to the measurement.

Also keep in mind the lobe profile needs to be conservative to keep control of the steel valves in that cylinder head. The 2.165" valve in it weights something like 113 grams! That's the main reason the PRC LS3 aftermarket castings feature hollow stem valves.

 

What does that matter?

 

You mentioned guide plates.

Since he plans to use stock rockers, guideplates wouldn't be used.