Here's a patch that I hope addresses the main problem. The basic idea is 
to avoid the confusion exemplified in Bug#30408 by changing Emacs so 
that it ordinarily signals an error if it reads a program that contains 
an integer literal that is out of fixnum range. However, if the 
out-of-range literal is followed by '.' then Emacs continues to silently 
convert it to floating-point; this is intended as an escape hatch for 
any programs that need the old behavior (I expect this'll be rare). 
Thus, on 32-bit Emacs, plain '536870912' in a program causes Emacs to 
signal an overflow while loading the program, whereas '536870912.' is 
treated as a floating-point number as before. (On 64-bit Emacs, the same 
two literals are both integers, as before.)

Unlike my previous proposal, this patch does not affect the behavior of 
string-to-integer. As I understand it, that was a primary source of 
qualms about the previous proposal.

I've tested this on both 32- and 64-bit Emacs on master. This patch has 
helped me to find a couple of integer portability bugs which I already 
fixed on master.