Stock lifters are fine to re-use. Pull the cam and look for markings, if not then buy a cam that you are comfortable with rather than using an unknown.
And yes, you HAVE to measure for proper pushrod length, there is NO other option and nobody will be able to tell you the right length except you and that tool.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.