Dave,
I took a couple of extra lifters and shimmed them solid. Then, I picked up a couple of checker pushrods from Trend. It's a bit of a pain as you have to install the lifters, then torque the head with the head gasket, then mount the rockers, then adjust the pushrod, then remove everything.

I like using Trend Pushrods as they have a great selection of stocked sizes/lengths. Once I have the pushrod measurement from the checker pushrod, I see how close they are to the stocking sizes with about .050" preload added to the measured pushrod dim, and pick from there. On my current engine, based on Trend stocked lengths, I think it worked out that I have about .040" preload. But that's how I do it.