Regarding the 8-ga link, don't limit your search to the Internet or "traditional" sources. Try a local electric shop, well driller, or electrician for the link. Some Class 8 truck dealerships may be able to supply a prefabbed 4-gauge cable with the 8-ga link already installed as a service part; speak with the service department electrician.
Adhering to your original plan:The alternator will not need to produce 140 amps for very long before the battery begins to recover and the charge current decreases significantly. As such, the factory firewall junction block should tolerate the transient current load as long as both 4-ga cables share the same 1/4" stud of the junction block. Or, upgrade to a 5/16" junction block. Be careful that the cable ring terminals are properly sized to fit the stud. Specify SGX battery cable due to its superior heat and chemical tolerances. Install
two fusible links: one in the charge lead where it connects to the junction block and one in the junction block-to-starter jumper where it connects to the 3/8" starter solenoid stud. Mechanically crimp and then solder the terminal ends to the cables using 60/40 or 70/30 rosin core solder. Then use quad-wall, adhesive lined, polyolefin heat shrink to insulate the 4-ga cable to its terminal end junctions. Route the 4-ga cable through
Thermo-Sleeve in the vicinity of the starter to protect it from radiant exhaust heat. Use 2-ga SGX stranded copper starter and ground cables to connect the battery to the starter and engine block.
An alternate direct routing to battery:Route the 4-ga SGX cable directly to the battery through an 8-ga fusible link installed at the battery connection. Optionally replace the 12-ga starter-to-junction block run with 8-ga SXL cable protected by a 12-ga link at the starter solenoid. If you anticipate the potential for supplemental B+ taps directly off the battery in the future, install a 3/8" (10 mm) junction block on the radiator support adjacent to the battery. Route the alternator 4-ga charge lead protected by the 8-ga link directly to the 3/8" (10 mm) junction block. Jumper battery B+ to that same junction block stud using a short length of 2-ga cable; the starter cable will still connect directly to the battery as before.