Why is the peak value(s) of the spectral radiance of the CMB different if you calculate with freq. instead of wavelength?
Because $B_\lambda$ is not just $B_\nu$, with $\nu$ replaced by $c/\lambda$.
The relationship between the two functions is that
$$ B_\lambda\ d\lambda = B_\nu\ d\nu$$
since one is defined in terms of flux per unit wavelength, the other as flux per unit frequency.
Thus
$$B_\lambda = B_\nu\ \left|\frac{d\nu}{d\lambda}\right| = \frac{c}{\lambda^2} B_\nu, \tag{1}$$
where you can then substitute $\nu =c/\lambda$ into the expression for $B_\nu$.
To find where the maximum of a function, you differentiate and set to zero. Because of the above, the wavelength where $B_\lambda$ peaks is not $c/\nu_{\rm max}$. We can see that by differentiating both sides of eq (1).
$$\frac{dB_\lambda}{d\lambda} = -\frac{2c}{\lambda^3}B_\nu + \frac{c}{\lambda^2}\frac{dB_\nu}{d\nu}\frac{d\nu}{d\lambda}$$
$$\frac{dB_\lambda}{d\lambda} = -\frac{\nu^3}{c^2}\left(2B_\nu + \nu \frac{dB_\nu}{d\nu}\right).$$
The usefulness of this expression is that you can easily see that if $dB_\nu /d\nu$ is zero at some frequency, then $dB_\lambda/d\lambda$ cannot be zero at that same frequency.