help with pushrod length

the best way to know is to buy a pushrod length checker. You put the checker where you would have your pushrod, and you adjust the length of the checker until the rocker stays on the valve correctly. This is the best way to account for milled heads/blocks, and other irregularities. Then order custom length pushrods. I don't know that I would want to use stock pushrods with guide plates. You want hardened pushrods.

Go with hardened pushrods for sure. I would also use a pushrod length checker and make sure to have the tip of the rocker arms on the center of the valves or you run the risk of mushrooming the head of the valve. Just draw on the top of the valve with a marker, then tighten down the rocker arm and see where it leaves a line, if it leaves it in the center you have the right length, and then you just have to order them.

The three different lengths are due to 1. roller cams use a short 6.25" length due to the taller lifters. The other two applications are due to the switch in april 66 to rail rockers and the required longer valve stems to gain clearance for the rockers over the retainers. But as the others have suggested the correct way to determine proper length is to use checking pushrods. You MIGHT get lucky with off the shelf lengths but you might not If you've got stock heads, valves and the block deck and heads haven't been milled too much, you may be OK with off the shelf ones. Only way to know is to assemble the engine and roll the crank around and observe how the rocker tip contacts the valve stem tip. The contact patch should be as close to centered as possible.

If you're using Chevy valves, then all bets are off. Get a pair of adjustable pushrods and figure what length you need.