"Why would the '73 B20E heads not be as good as the '70-72 heads?"

The actual BEST choice is 1973+ B20A

From around 1973, or whenever the previous batch of heads run out, the exhaust port changed & I think there is a tiny bit more meat in the inlet port in the right place for porting.

EXHAUST PORT: This is the biggest choke point in B18 & B20
If you sit the head flat on a bench, then sit down and look in the exhaust port at the FLOOR from the manifold face to where it turns down to the valve, the early heads rise up about .125"/3mm. In the 73+ heads the floor raises up about.25"/6mm. This lets you make a much better Short Side Radius(SSR). If you can't weld, then this is a big advantage. Most people hack away at the LSR which actually needs metal added for most purposes. In the beginning, I only had one choice, I welded up the floor to make it about 10mm higher inside, gives a great SSR. Using a near stock B20E valve I got the flow up to 80cfm@10"H2O which is roughly 133cfm@28". Both early & late heads are so hit & miss that the flow is around 60cfm max. (6 banger B30A/E exhausts, which are round, max out at 55cfm.)

INLET PORT: The E head needs nothing done to it if you are only lifting the valve to .25D, just careful seat, bowl & chamber work. This is a quick & easy IF YOU HAVE all the tools. Works fine with Twin 45DCOE & the stock injection manifold.

BEST INLET PORT: Starts with an A/B head because the port is smaller & you can leave metal in places where it is already gone from & squeaks up the mid-lift flow a little & the velocity too.

The A head is thickest, so you keep those for Stroker usage.

"Volvo Owners Club engine chart shows the same specs for all 3 years."

That's because they read it from a book. In 1974 the B20E cam changes from D to K, which looses approx 5 BHP peak, but, the K cam with a stock or mild head is equal to or better up to 5500rpm & much kinder on the valvetrain.

The graphs shown by Volvo are 'artist interpretations'. Although it's a very average intake, a B20E/F should still show some kind of double hump in the power/torque curves.

In one 'good' source, there is an B20E that makes 135BHP, it's a misprint, but it drives some people nuts looking for the specs of this unicorn.


ENGINE BUILDING: Do you have a spare engine? Bolt it up to engine stand & get the stuff you need to degree in a cam & play with that. If you can't master that skill then you can't built even a stock engine properly.