Shvetsov family tree

\documentclass[10pt,border=1pt,crop,multi=false,tikz,class=scrartcl]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage{setspace}
\usetikzlibrary{calc, positioning, arrows}
 
\renewcommand*{\familydefault}{\sfdefault}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}

\tikzset{
	vbox/.style = {draw, rectangle, rounded corners, text centered, black, text width=7.5em}
}
 
\makeatother
\begin{document}
\begin{tikzpicture}[node distance=3.75cm, thick, >=stealth']
	\node (a1) [vbox] {Wright cyclone\\ R--1820\\9-cylinder air-cooled radial,\\ca.\ 1931};
    
    \node (b2) [vbox, below = 1.2cm of a1] {Shvetsov\\ M--25\\ 9-cylinder air-cooled radial,\\ca.\ 1934. 13,888 built};
    \node (b1) [vbox, left = 1.5cm of b2]  {Shvetsov\\ M--80\\ 14-cylinder twin-row, air-cooled radial,\\ca.\ 1939. Cancelled};
    \node (b3) [vbox] at ($(b2)+(3.5cm,-1cm)$)  {Bazarov\\ M--65\\ 9-cylinder air-cooled radial,\\ca.\ 1939. Cancelled};
    \node (b4) [vbox] at ($(b2)+(7cm,0)$) {Shvetsov\\ M--70\\ 18-cylinder, air-cooled radial,\\ ca.\ 1938. Cancelled};
    
    \node (c1) [vbox, below of=b1] {Shvetsov\\ M--81\\ 14-cylinder twin-row, air-cooled radial,\\ca.\ 1940. Cancelled};
    \node (c2) [vbox, below of=b2] {Shvetsov\\ M--62\\ 9-cylinder air-cooled radial, two-speed GCS\\ca.\ 1939. 40,361 built};
    \node (c4) [vbox, below of=b4] {Shvetsov\\ M--71\\ 18-cylinder, air-cooled radial,\\ ca.\ 1942. Cancelled};
    
    \node (d1) [vbox, below of=c1] {Shvetsov\\ ASh--82\\ 14-cylinder twin-row, air-cooled radial, two-speed GCS,\\ca.\ 1941. 57,898 built};
    \node (d2) [vbox, below of=c2] {Shvetsov\\ M--63\\ 9-cylinder air-cooled radial\\ca.\ 1939. 3,087 built};
    \node (d4) [vbox, below of=c4] {Shvetsov\\ M--72\\ 18-cylinder, air-cooled radial,\\ ca.\ 1945. Cancelled};
    
    \node (e1) [vbox, below of=d1] {Shvetsov\\ ASh--83\\ 14-cylinder twin-row, air-cooled radial, \\ca.\ 1945. Small numbers};
    \node (e2) [vbox, below of=d2] {Shvetsov\\ M--64\\ 9-cylinder air-cooled radial\\ca.\ 1939. Cancelled};
    \node (e4) [vbox, below = 1.5cm of d4] {Shvetsov\\ ASh--73\\ 18-cylinder, air-cooled radial,\\ ca.\ 1945. 14,310 built};
    
    \node (f1) [vbox, below of=e1] {Shvetsov\\ ASh--84\\ 14-cylinder twin-row, air-cooled radial, \\ca.\ 1945. Small numbers};
    \node (f2) [vbox, below of=e2] {Shvetsov\\ ASh--21\\ 7-cylinder air-cooled radial\\ca.\ 1946. Cancelled};
    
    \draw [-stealth] (a1) -- (b2);
    \fill ($(a1)!.5!(b2)$) circle [radius=2pt];
    \draw [->] ($(a1)!.5!(b2)$) -| (b1);
    \draw [->] (b2.0) -- (b3.180);
    \draw [->] ($(a1)!.5!(b2)$) -| (b4);
    \draw [->] (b1) -- (c1);
    \draw [->] (b2) -- (c2);
    \draw [->] (b4) -- (c4);
    \draw [->] (c1) -- (d1);
    \draw [->] (c2) -- (d2);
    \draw [->] (c4) -- (d4);
    \draw [->] (d1) -- (e1);
    \draw [->] (d2) -- (e2);
    \draw [->] (d4) -- (e4);
    \draw [->] (e1) -- (f1);
    \draw [->] (d1.south east) -- (f2.north west);
    \draw [->] (c2.east) -- +(1cm,0pt) |- (f2.east);

\end{tikzpicture}

\end{document}