build my stock rear or go for a 9 in

It depends on several things, however you will need less with a 351 than most folks use in their 302 cars.
The 351 doesn't need to turn as high, and won't turn as high as the short stroke 302.
You cam specs, tranny ratios, intake, carb, car weight, tire sizes, etc... all play into final gear selection.
I think the most important number you should consider at this point is the first gear ratio of that 4 speed you have.


Scooper
The tubes are fairly critical to get lined up with the housing straight.
If you are not straight, you can run into problems with axle bearing failure.
The best bet is to have someone that narrows rears do the welding. They would have the jig to keep everything lined up. There isn't much to the jig, but it's more than most folks have laying around.
Dave

 

Currie Enterprises makes a real nice 9 inch. Actually they make several depending on what you want to do and how much you want to spend. I got a 9" with 3.89 gears and a TSD (torque sensitive differential) with explorer disc brakes (calipers and all) for less than 3 grand (albiet not much less). It was a direct bolt in. The only mods I had to make was to the head cable on the E-brake(the axle come with the rear cables) and add an adjustable proportioning valve. I even used the original driveshaft (with an adapter u-joint). So I guess that's 3 mods.

 

I don't like much over a 3.73 or 3.89 gear if I'm gonna be driving it.

 

With you setup Id go with the 9"..............