you can also buy gears Brand new for the 8-1/4 dirt cheap..
i still have a set of 4:10s laying around from yukon,i think they cost less than 200.
the key to those rears is to rebuild using Spacers instead of a crush sleeve.
buy the aftermarket kit of Spacers,that has all diff sizes then use those instead of the crush sleeve.
with the factory crush sleeve,you get 1 shot at getting it right,correctly torqued to set.
whereas with the spacers,it will be easier to get right and Stay right because theres no sleeve to fail.
as far as your motor goes,from the start figure out the condition of the motor,ie timing chain,plugs water pump valve stem seals,compression for each cylinder,etc etc.
after that,either get your distributor recurved or buy an aftermarket performance<< one,thats curved for your Application.
ditch that holley you have,and look for either a street Demon or an edelbrock.
if you are going to street cruise this car,then get a 625
if you plan on beating it senseless then go for the 750.
heres a link to the Demon 750,and keep in mind this is Next Gen version of the Thermoquad design too.
https://danaport.com/products/demon..._content=sag_organic&utm_campaign=sag_organic