We've been trying to get my new motor set up with my old B1 MC heads and the rocker gear they came with. I bought a new cam that has .500" lobe lift. The lift at the valve should be .850 with the 1.7 rockers. When we put the valve gear all together with the 850 lb. Manley springs we are only getting .750 lift at the valve on the intake and about .780 lift on the exhaust. When we put it together with lightweight checking springs we had .850 intake and .875 exhaust.

Thats a LOT of deflection!

There are only 2 places that have a lot of compliance, and only 1 that's easy to fix.
How big are your pushrods?

Pushrods are 3/8" .120 wall from Smith Brothers.

Aren’t you running the Koffles rockers arms and shafts? If so you will need spares I have a full set with spares. Looks like you will need better pushrods. I’m running .145 wall 3/8 pushrods the 7/16 would be better.

I just bought a set of 8 Manton 7/16 to 3/8 single tapered .167 wall pushrods for my 527 440-1 headed bracket motor intake rockers. I originally had a set of 7/16x.095 series 3 made but the intakes where rubbing on three cylinders And I ordered them a little to long No more 3/8 diameter pushrods for me on any roller cam motor that has more then 600 lbs open pressure I picked up 8 HP and move the peak HP up 400 RPM on the engine dyno by switching from a set of Smith Bros. 3/8x (supposedily SP?) .120 wall(they actually measured .083 thick after one of them broke in half during a dyno pull ) to a set of Manton 3/8x.120 Series three pushrods Time to move up, let me know if I can help, I'm a dealer for both

We run the same Smith bro push rod with 650 open press, @ 7.50 long, I would think you need 7/16 @ your length

Is this a single shaft system... you most likely have
a few different places flexing.... the push rod, the
rockers and the shafts
EDIT
Forgot to add the cam itself flexing but your not running
big spring pressure... whats the cam diameter

Are both heads on & valve train lashed?

did you check for spring binding? bring your lift to the top, use a feeler guage to check..050

I agree the shafts probably move, but it can't be much or something would give up.
7/16" is going to help a lot more than it sounds, it's worth doing if only to remove (or at least minimize) 1 variable.
Taper is just as strong as single diameter, since the "bendy" part is mid-point (the ends could be 1/4" for the last inch without penalty).

FWIW I run Manton Pushrods. They are 7/16"x.168" wall. I have about .020" deflection in mine. Lobe lift is basically the same. I also run a lot more spring. Mine are 1010 open. Also I run T&D paired rocker gear.

On my last B1 original motor I ran a 3/8" Smith Brothers pushrod with a little less lobe lift than you and say about .040"+-. I also ran the manley 850lb springs. This was with T&D common shaft rockers. Could have been the sommon shaft or maybe it was the pushrod??

Had to go check over my notes but that is what I found.

The single shafts are a PITA to upgrade.
IIRC OD is 7/8"?
The OD is far the biggest factor, but needs everything changed (stands, rockers).
Comparo, with 7/8" OD with 1/8" wall as 100%:
1" OD with 1/8" wall = +57.7%

The material doesn't matter at all - moly isn't less bendy than mild steel and alum is worthless.
Wall thickness is what's left, and it doesn't do much (although it's cheap and there's no down-side). You don't even really need it hardened or chromed if you have bushings (needle bearings or plain rocker ID: absolutely necessary).

Comparo, with 7/8" OD with 1/8" wall as 100%:
7/8" OD with 1/4" wall = +30.6%
7/8" OD with 3/8" wall = +35.1%
Give me your wall thcikness and I'll do the math for you.

Another stand between the existing might do something, but not much room.
Upsizing all the stand hardware will improve location (i.e., 5/16" > 3/8", etc.) but the heads have to be tapped, not much room in the stuff, etc.
Extending all stand/cap hardware upward and using a fabricated tie-bar will do something, but the obvious problems:
1. must clear the rocker path, so tall
2. no clearance in the cover
3. must be reamed to accept stand studs

Sure I'll give you a stealing deal on my stuff it includes pushrods too. Went with T&D.
Email me jkcomp5957@yahoo.com