Rich,
The Mexican 302 is really not a Hipo stand in. The only physical similarity is the main caps. The Mexican main caps come up fairly often for small money compared to the short block. There are extra bosses cast into the front of the block on both sides which are a dead giveaway of its origins. Standard 302 rods are shorter than 289 rods and, yes, they have 5/16" bolts. Ford shortened the rod to make up for the longer stroke of the 302 but used the same pistons for both engines. The only rods that are the same length as the 289 is the Boss 302. Generally, when you disassemble a high mileage early 302, you will find at least one broken piston skirt. This is due to the tight angle between the piston and rod with the shorter rod. I hope that the one that you are thinking of is local to you as shipping can be a nightmare.
This is all just my personal opinion and that with about $7 will get you a Starbucks coffee. 
-Fred-
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