I need to order pushrods for my 292. I have a couple of questions for the group:

1. With a 3/8" pushrod, are the ends still 5/16"? Do I need to get special lifters or specific rockers or anything?

2. On the SBC's I have built previously, I just used the moroso plastic thingy that told me how long to get the pushrods. How do you figure out the right length when that tool is not available?

Without having some specialty engine building tools to use, its going to be difficult to estimate how long to make replacement pushrods. An adjustable pushrod would be a big help for you in determining this. Many racing pushrods i've used in the past have been tapered, and have different sizes on each end. You should only need to get a standard straight wall pushrod, but larger diameters may require you to grind for pushrod clearance at and around the top of the deck area, and the cylinder head. So just make sure you rotate the engine over through several cycles if using a bigger OD pushrod to make sure you have adequate clearances.

56er,
Are you close to stock. or running a big lift cam and big springs? planning on using the stock rocker arms/press in studs, or did you machine and install BBC screw in studs for roller rockers? all these things affect what you need.
I think if your closer to stock you can stay with the 5/16 pushrods and not have to machine the side galleys and head to clear 3/8 pushrods.

Oh, and instead of a plastic thingy, go to Comp Cams and order an adjustable pushrod/measuring tool ($15 and up)
go the the tech help and search for pushrod measurement.

I have recently, taken a stock pushrod, cut it in 2 pieces, removed a extra 1/4", threaded the interior on both pieces and used a threaded rod inside to be the adjuster. Takes a real long caliper to measure it though. Tom

The pushrod ends I have ordered to make my own 3/8 pushrods have 5/16 ball ends.

To determine correct pushrod length set the valve up with a light checking spring. Put the cam in on the base circle (TDC). Make a sample pushrod like tlowe suggests above, using zero lash if your rocker arms are adjustable. If using fixed rocker arms, compress the lifter plunger to half of its total travel. Install it and turn the engine over through two revolutions, measuring valve spring compressed height for signs of coil bind (on the springs you plan to run, not the checking spring - duh!) and also check for valve-to-piston clearance. It is also a good idea to check the rocker arm relationship to the valve stem. At the midpoint of your camshaft lift point this angle should be 90 degrees. This may require some slight adjustment to your length for optimum effective use of your camshaft. Also, a little thread locking compound on the threads of the adjustable pushrod helps tighten up the ends of the pushrod so it holds its length better. I hope I did not leave anything out. Good luck.

I use a height gage on a granite plate to determine precise length.

Thanks, guys. I don't believe they sell a pushrod length checker for our length pushrods, so I'm going to make this thing straightaway.

Heater, the current cam I have is .545 lift, but I am considering changing it. I have 1.8 ratio BBC rocker arms and 7/16" screw in studs. The springs are spec'd for the cam and the boost+n2o I will be running, but I don't really know their specs. I just told Mike what I was doing, and he ordered the springs and assembled the head for me.

You're introducing a lot of new dimensions into your rocker geometry, which makes predicting the pushrod length more difficult.
As FF said above, I'd prefer to get a theoretical line between the rotational center of the rocker (the radius of the ball + the wall thickness of the rocker bottom, measured from beneath), and the intersect point between the rocker pad and the valve stem tip to make a right angle to the valve stem axis @ 50% valve lift = (.545" - lash) ÷ 2. Correct pushrod length is the result result.