A couple of thoughts. You first should decide on what you want to do with the car. The gearing should be setup to match your engine RPM range and trans.
With your 77 you most likely have a base 350 TH350 and 308 gear. Pretty much a slug on the streets back in '77 and today. Nice for cruising around and traveling.
There is very little difference between a 308 and 336. In fact I wouldn't recommend rebuilding a diff if that is the only concern. I changed from 308 to 336 in my 72 350 automatic and there was really no difference in driving, maybe a little more off line but no tire burning difference. The RPM @ 60-65 mph ia about 2500-2800- my speedo is off 3 mph with the change because I didn't bother to change the speedo gear yet.
You would feel a difference with a 355 but more so with a 373. I use 373 over the 370 gears. You'll probably want to go to a 2400 stall TC with the 355 or 373's. You will feel a difference then. Add a cam,intake,heads, and 2.5 real dual exhaust and you're going to feel a big difference. Tune and jet the carb and set the timing and go.
It will be in the 3200-3400 RPm 65 mph range and gas mileage will be about 10 mpg if that matters. My car, out of tune, 336's, cruising got a best of 15 mph watching the pedal. The only way you're going to get much more would be to add a OD and maybe FI. If an OD is something you may go to then I would use the 373 ratio over the others. I'd say just about all the diff's I've built for use with an OD were 373's.
Now as far as a build you have several options. You could go a catalog exchange/rebuild. Those will usually cost more then they quote because they low ball them and figure just a master kit and maybe clutches. Some places go evenr futher by cutting corners using cheaper parts and lousy work practices. I could write a book on that subject alone and have the pictures to prove it.
Next you could buy the parts and give it a go yourself. I have coached a few guys through the work but if you never worked with a micrometer, dial indicator,or have access to a machine shop I wouldn't recommend it. Yes plenty of places rebuild them without a lot of that stuff, a lot of those end up here with me to correct.:smash:
I could build you a diff to handle up to 1000 hp as well. I build them to application and they are second to none.
As with anything it all comes down to how much you want and have to spend on the work. Do you want average or the best?
77-79 diff's had issue with the side yokes as Gene mentioned, also used weak clutches,and RG bolts. The big problem with a lot of the RG bolts is they went to a flanged head bolt and didn't loctite them in place. I wonder if they even torqued them becaue a fair amout of those diff's had RG bolts back out and jam up the diff's. The good point, they can be built very strong using Tom's parts and good machine pratices.
Good luck
