Alot of good useful combined info here. I will add a little bit to what Bud posted.
Ive used almost any rad. in a F within practical limits and have found the only real limits is the distance between the frame rails and the distance from the base to the top of the core support.
One of the best things of being in the fmj world is we can use nearly any drive line configuration from the muscle car era to far past it.
Maybe Bud can articulate a bit further on the strap aspect as he mentioned as I see it as useful if one was doing a proper resto or conversion and myself have never thought of it simply because for myself I never found it necessary but now im wondering what I might of done if I had of done it...hmm.
With the F-s (at least early ones, 76-77) having two core supports, ac and non ac and to further backup the use of the older radiators if we look at the early 76-s they use the older type a body rad. vs the long slender that the F-s went to later and used in the M and J cars.
Here are a few examples in the pics. One beater I have has a early 70-s (74) BB (400) B body rad, I dont recall the PN but its actually the max airflow unit for a BB ac car. Yes, the core does overlap the non ac core support but factually causes no issues and on the reverse side still gets full effect from the oem shroud.
Its a bit ironic that so many who have seen this car in person have never caught it because it looks so factory because it utilizes oem parts, just a bit older. Only a few FMJ guys have ever noticed and said "Where did I get the BFR" at.
The second pic I robbed from a current ebay listing of a BB 440 RR, but was a 400 car and the lazy F--- dont even have a shroud on it asking 29K Canadian. But its the same unit I have.
I now have a proper mopar hose on it but in this pic its got a flex hose.
The third and fourth pic shows the early 76 F-s with oem correct radiators and you can see they are not even the full width of the core support opening and still the older design.
As far as fitment, yes flush is good but if using a larger rad core then opening touching is bad, 1/16 hard mounted is good, simply round stock cut to the needed thickness is the easy ticket.
Not to get deep into it, a simple 440 iron headed F can live with ac and a factory /6 rad with 3.55 gears.
Ive used several Al rads from $700 dual pass cross flow units to the 2 and 3 core (1 inch tubes) sold off ebay for under $200 delivered and have never had a problem with either. I do stay away from any plastic config replacement units.
There is alot of confusion for many when we get away from the oem iron head copper core rad vs more modern al heads and al rads because from a thermodynamic standpoint were trying to merge two worlds, old and new.
On alot of stock stroke small blocks there are still many good iron head/piston choices choices that still provide quench and other then weight a little diy work goes along way with good results, but thats a whole different topic.