Jim - the whole install is shown in the
Accessories Installation Manual .... you can find some
pics here of the heaters - the firewalls had the locations marked for tubes of the 2 different model heaters, and the dash mat should have "knock outs" in those locations if you still have the original, the heater sets it's own location because of the piece that attaches to the kick panel opening, start from there .... the cables and wires really can only go where their length allows, pretty straight forward .... also, there are some "stand offs" not shown in your pics that go on the mounting studs to level the unit to the firewall and line it up to that kick panel opening, short pieces of 1/2 copper plumping pipe work
Bill