Can be done, never did hyd flat but many solid flat tappet yes they need to oil thru the pushrod. Most I did were Hughes, lifters and pushrods. Some heads will need to be clearanced were the pushrod goes thru the head on the back side of the intake mounting surface because the lifter is shorter. Can’t remember if it was more so with the factory head or the EQ. Because of rules I had to run factory rockers, rollers could be different. Pretty sure Hughes had pushrods made up that are in his catalog to make this work, I know they weren’t custom, matter of fact I usually ran his adjustable pushrods for lash adjustment, I did some roller hyd regrind cams where the base circle was smaller and I ran the thin (.029”) ? head gaskets so the factory pushrods worked.
This was afew years ago so I’m shooting off the hip, pushrod clearance is a little foggy, pretty sure it was the back side of the intake rail, we did try all kinds of different rockers to try to make everything work better (geometry) ended up the factory ones worked good.