The header will fit.
The B20 gives you 200cc more piston displacement and better possibilities
of cylinder heads. The B20 combustion chamber is too big for the B18
unless you bore it out to 3.5" and that is about as big as you want to go.
You can go to 3.622 or more with the B20 block. The B20 heads (at least
3 different ones) come with bigger valves and MUCH better valvestem seals.
They usually had bigger oil pumps also. Nearly all the parts from the
6-bolt versions bolt together OK and quite a few from the 8-bolt (74-75)
versions will work in the right combinations.

I much prefer the B20 because it gives you a headstart on better performance.
--
George Downs, Bartlesville, Oklahoma, Central US