You asked what is MY approval basis for installing a 35A generator. I didn't install it. Cessna installed the 35A system when my airplane was built. (SN25713 left the factory with factory camera and agricultural kits installed, which included the 35A gen system. It is listed in the original Equipt List.)
You address a question implying that the change from a 12 or 20 or 25 ampere system to a 35 A system is a minor alteration requiring only a log book entry, but FAR 43, Appdx A states under para (a)(1)(xii) that the change of the electrical system design is a MAJOR ALTERATION, so it would require a Form 337. However, no need for Field Approval (Block 3) is req'd.
The IPC is not an approved document so that corresponds with Bruce's comment as well as may explain why you found no IPC illustration to support gen cooling or blast tubes. But, you might find it interesting to view Fig 60 in the 172/175 IPC which clearly shows the cooling hose, blast tube generator cooling installation. Here it is in PDF format.
The TCM X30013 Ovhl Manual, Pg 54 paragraph you posted states a cooling blast tube "must be connected to a source which will maintain a minimum of 1.5" (cooling air ) across the generator. The Service Manual instructions for removing the generator states (para 17-28 (b) "loosen the clip securing the blast tube and pull the tube clear of the generator." This can be done without disconnecting the cooling-air supply-hose...a plausible explanation for why they did not mention the cooling hose.
Yes, there are a lot of details which require familiarity with aircraft service and repair which are not perfectly documented. This is why FAA has minimum experience requirements for repairman certificates.
An example of the need for such experience is found in the AK172-6A Instructions you mention. If you read it closely, you will notice that the instructions have the workman remove the lesser-rated generator and replace it with a higher-rated generator....but make NO MENTION of increasing the wiring gage/capacity to accommodate that upgraded system. This is likely because the 172 (in the serial range applicable for that kit) left the factory with a standardized wiring harness which already had the capability to handle the "optional" 35A generator/regulator. All that would then be necessary would be to also upgrade the fuse/c.b., which the AK172-6A does instruct.
THE PROBLEM WITH USING SUCH KITS on other models aircraft...SUCH AS OUR 170 MODELS... is that our airplanes left the factory with wiring appropriate for the generator system installed AT THE TIME OF PRODUCTION. This means that if the AK172-6A is used to change a 12 or 20A generator in a C-170 to a 35A system...the wiring will likely be TOO SMALL gage. Therefore additional changes must be made to safety convert our 170s. While I am relying upon memory in making the next statement, ... it is my belief that the smaller gens in the 170 were not originally equipped with cooling-air supply, and that the upgrade to a 35A gen made 3 mandatory changes in the upgrade: Increased wiring and fusing capacity, cooling-air/blast tube system, and dampered crankshafts.
As those items became standardized in later aircraft like the 172... they were simpler upgrades. Your mention (regarding the TCM paragraph quoted) that TCM required the 12A gen to be cooled overlooks the date of publication of the TCM X30013 manual. The paragraph was found in the June 1982 manual and perhaps some earlier versions, but will not be binding to airframe items installed separately. TCM is not the certificate holder for Delco-Remy generators. TCM, under a marketing-system known as "specification", in which "spec numbers" were assigned to engines PRE-configured for particular airframe installations, may have included generators AS WELL AS GEN COOLING SYSTEMS to the customer's order. In such cases they might offer instructions for engines so-equipped.
However, generators, pumps, regulators, and many other accessories are usually considered AIRFRAME items... not powerplant items. Therefore the Airframe mfr's manuals apply in those instances. I would not consider TCM's manual binding with regard to generator installations, however helpful the info might be.
You ask about the 50A generator option. It is not possible in a 170 because the 50A generator is too long... the firewall is too close to allow the installation. The later 172/175 firewalls were "stepped" and could accommodate the longer 50A generator.