That is a good price for the Harland Sharp and they have a good reputation on the Corvairs.
Comp Cams and Crane both sell what they claim is for the inline 6, but I'm not sure they are not the exact same as the 396-454 rocker.
I have the Crane Gold in my 292, but I think I should have used the Crower Stainless roller rocker.
Crower
In the left menu, click on SS & Aluminum stud-mount rockers.
They have 3/8" and 7/16" and a true 1.75 ratio, which tells me they actually made them for the inline 6.
250 292 rocker and price

They should work under the stock valve cover.

'67 GMC 3/4 292 4spd