TB Bronco 42” mower deck install on TB Pony

Sounds like you may have to shorten that front adjuster rod. 
 
Weather cut and re-weld, or bend / modify.
 
You adapting two different machine parts together, so some engineering is likely.
 
If you have access to "Tractor Supply", they have many deck belts,
in 1" increments that should help in getting the exact size you need
(return the ones that don't fit)...
 
 

Sure, that'd work..
 
**For testing purposes ONLY, cut some .5" PVC PIPE, (approx 1-3").
Slip it over that threaded rod BETWEEN the adjusting nut and the frame.
That'd mimic a shorter rod/more threads on your current rod and see how 
that mod works.
 
 
 
 
.

There's no pictures posted..
 
I just had a customers rider in for repair, also a 42" two blade.
Besides the running issue, he said the "blades didn't work well".
 
Further inspection showed the belt 3-4" TOO LONG (same as yours).
Ordered a new belt, same size.  Double checked another site, it should be the correct belt
but was again too long.
 
I pulled the belt tensioner about half way tight, then ran a wire (rope would be better)
where the belt routed and marked the length needed. 
Made a trip to "Tractor Supply", where they have a measuring device and numerous belts.. 
I believe it was a 91" I ended up using and returning the rest. 
 
As I re-call, the machine called for a 95" (or so) belt..   
 
So what that issue was about, dunno.  The deck DID belong to the machine but this belt fixed the issue.
 
.
 
.