New World Datsuns...

Soon I am going to be motivated enough to drop L20 with W58 peanut head in the 521. Both manis from the 16 will be going onto the 20. It will be an L20 with L16 exhaust mani 32/36 weber and the Offenhauser intake. I have heard that the Offy was made just for the L16. Does anyone have info on this?
What will it be like on the L20? Too small or will I even notice a difference? Someone told me I may lose some top end power. I just want a dependable little hwy cruiser. Not looking for massive power upgrades or anything. That and I don't have a stock L20b intake

The L20b was the greatest upgrade I ever made in my 521, the rest can be dealt with, there is no substitute for displacement.
As long as the Offy manifold intake ports are not bigger the head intake ports, you should be good.
The Weber can be re-jetted if needed.

Offy intakes usually are dual plane. I had one on my L16 (came with the car) then 18 (then I had SUs).

I got pretty good gas mileage on the L18, 28mpg on a pure HWY drive with a few times hitting triple digits.

It may or may not be more restrictive. They were tuned for the L16 to my understanding. Seems they were for low end power, but could choke on the high end.

Probably depends more on where your RPMs are on the freeway. Like my trip I was probably driving at 70-80mph most of the trip, and was staying below 4000rpm with a motor that is only 90cc less than an L20b.

I would probably just stick with a stock L20b manifold with an adapter and block the EGR. Or a ported L16 manifold. But in reality do what you want and it will probably be fine.