Recently, I sold my 73 sportsroof and picked up a 72 mach 1.
the car has been smacked in the rear, and it has been pretty much reassembled using a 67 parts car.
it has an 8 inch rear end in it, and I'vebeen wanting to put a POSI, but I know you can weld up the axles because my friend and I did it in his truck.
the only problem is his truck is simply built for mudding, and the truck is never driven on concrete except for loading and unloading on the trailer.

all I'm wondering is if the 8 inch could handle this, and what the downfall would be to do it. and is it even capable of being driven on the street, or is it simply for track?

 

The rear tires need to be able to move at different speeds if you want to drive the car in anything but a straight line. Period. Permanently locking the axles together will make the car completelyunsafe on the street. The tires will chirp around every slow corner (wearing out the tires), and the *** end will slide around every fast corner.

If you want both tires spinning, find yourself a decent limited-slip unit, or even a locker or torsen setup.

 

dude just install a good ole' fashioned 9" rear with so detriot lockers hehehehehe

 

See that's what I thought but I wasn't sure.
which is why I thought I would ask before I went and did it.

Eventually I was going to get a 9 inch with POSI
but for temporary reasons until I can afford one, I figured maybe I could lock it up.

any other ideas that are cheaper?

 

You don't really need a LS diff. Save until you can afford something decent, you'll be happy you did

 

What kind of engine do you plan to run? Unless you're going to be racing and placing a lot of shock on the rear from a 450+hp engine, the 8 inch will handle what you want to do. You can get anything from a Trac-Lok to a Detroit locker for the 8 inch. You can't get Posi, though...only GM cars have that.

 

well we paid $1200 for the car
and it had a 289 put in it from the parts car.

but as soon as we got it, we pulled the 5.0 out of my 83 GT.
the engine may be putting out 350 horses for it has aluminum heads and an edelbrock roller cam. but I figured the 8 inch was more than enough, however I would still like a 9 inch to put under it for I plan to put the car back to somewhat of an original mach 1 aside from maybe modding the 351C it should have came with.

 

If it has an 8 inch, it doesn't have a 351. What's the engine code?

The 9 inch takes more power to run and is heavier.

Mach I's came with 8 inch rears in the base models.

 

the eight inch is not the original rear end.
the car was hit in the rear drivers corner, and the frame was wasted.
so the owner pulled the rear end, tranny, and engine out and sold the car
the previous owner before me bought it and used his 67 which was hit broadside and installed the 289, C4, and the 8 inch rear end. they cut and redid the frame, then they frakenstined a quarter panel in which has to be redone.
that is the way I bought it. I then pulled the 5.0 out of my 83 because the tranny in it was bad and the t-tops leaked so the entire inside of the car had to be replaced, and put the 5.0 into the mach 1.
and that is how the car sits now.

 

Colorardo is right and welding the spider gears is only good for a off road/race only.In a street car its not safe normal turns would eat up your back tires fast and in the rain or snow youll spin out easy.