Dick, from research I have done for the '64 Kansas City and Baltimore cars (Rad Manifest Codes found on the PHS docs) and knowing what the '64 Tempest Inspector's Guide calls out for Radiator usage as of the Dec. 19, 1963 compiling Engineering Releases thru that date, plus looking at the '66 MPC Rad usage listing, I can tell you that I have seen a variety of usages associated with the Tri-Power cars and I don't know if anybody will be able to say what was done with certainty throughout the year.
At the time of the printing of the Inspector's Guide, there was a Rad listed for 4 bbl 389 usage without A/C, code 57 with Trans Oil Cooler, 17.5" x 2" with .18" fin spacing. The Tri-Power Rad, also with Oil Cooler, was listed as the code 58, 17.5" x 2.63" with .18" fin spacing. This same Rad was used with the 326 or 389 when A/C was ordered.
There was no listing at that time for a Manual Trans specific Rad without Trans Oil Cooler. However, Rads for that application were released at some point and used. These were code 50, fin spacing unknown, but same 17.5" x 2" and specified for the Man Trans 4 bbl w/o A/C. The code 59, 17.5" x 2.63", fin spacing unknown, was specified for the Man Trans Tri-Power w/o A/C and also was used with the 326 or 4 bbl 389 with Man Trans and A/C.
The Auto Trans Rads could be substituted for a Man Trans application, the Oil Cooler ports were left plugged, but of course, the Man Trans Rads could not be used for an Auto Trans application. Presumably the Man Trans Rads were released because they were cheaper w/o the Oil Cooler.
I have documented one very late built KC GTO with Man Trans & Tri-Power that shows the Rad code as 50. This may have just been an oddity put together at build out that "violated" the Rad usage rules because of availability. Or maybe it indicates that there was a change in usage specified during the year.
I do not know if a Heavy Duty Rad option was generally mandatory when 3.90 gears were ordered in a 4 bbl or Tri-Power. However in the only '64 GTO (Fremont built) that I have ever documented that was ordered with the Special Equipment code 8 close ratio M21 4 speed, the original Window Sticker (not a repro) for the car listed all of the mandatory extra cost optional equipment that was known to be required if the close ratio (a very late year release for '64) was to be ordered. That extra equipment included GTO option, Tri-Power, 3.90 Axle Ratio with Safe-T-Track and Metallic Brakes (code P axle) Heavy Duty Radiator, and Heavy Duty Fan. The close ratio was never mandatory for such a combination, but if the close ratio was ordered, all of the rest was mandatory.
The latter two options are strange because there is no indication that Pontiac released a specific Rad considered to be HD for this application. My speculation is that it was the code 59 Rad but with a shroud and seals that may have been omitted except on A/C cars. In other words, nothing special about the Rad itself, just the added pieces to improve cooling efficiency. I have not even tried to understand what was special about the HD Fan. On the Window Sticker, the HD Rad was listed at $12.91, the HD Fan was listed at $3.12. Sales codes were 432 and 644 respectively and could be ordered on the All Series Special Equipment Order Form.
With the exception of the one very late built Tri-Power that seems to have been built with the 2" Rad, generally I have found the T-P GTOs to have gotten the 2.63" Rad as standard. So I figure the "Heavy Duty" Rad option only affected the shroud and seals when it came to the T-P application, not the Rad itself. Just my opinion.
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