Gt40p Pushrod Question 500/510

This may help.
http://www.hotrod.com/articles/setting-pushrod-length/

Nope, buy a pushed length check tool and order the right ones the first time.

He wanted you to measure P to V.
He wants you to check for piston to valve clearance.

The camshaft, whether it be the stock one that came in the engine, or an aftermarket unit doesn't dictate the need for a different pushrod.
In other words, if all you did was change out the cam and upgraded the springs,...nothing changes. Only after you take the steps to really upgrade things, does that need even arise.
Increasing spring rate may require a better pushrod, but it should still be the same length as the stocker was that it replaced. Typically,..a better pushrod means a chromoly or hardened steel piece,..a larger diameter, or maybe one that was built as one piece w/o the usual ball ends pressed onto the shaft of the pushrod.

Only after you start changing things beyond that will you have to go through the steps to measure pushrod length. Milling the heads, changing the head gaskets for a thicker/thinner unit, Different heads w/ roller rockers, the use of guide plates,..all require that you either check the length of the pushrod, upgrade it's diameter, change the length, or change it to a hardened unit because of potential wear related side issues.

Not just because you changed the cam.

Checking P to V clearance is a much more involved process. It can be done with the head on as is, but you'll need a lot more experience than you currently have to do that. Read here:http://www.hotrod.com/articles/hrdp-1303-checking-piston-to-valve-clearance/