Looking for a crate 350

[69lnghrn]

Hey guys! Well it looks like its time to buy a NEW 350. I finally ran mine to the ground. Its for my 69 Longhorn. Im still gonna be working in this truck and using it as my daily driver. Alot of people are telling me that I need a 4 bolt main because of the load I put in it and the things i haul. I dont plan on loading it as heavy as I use to, but there is diffidently gonna be a skid of plywood, couple squares of shingles, or a few kegs of asphalt back there at any given time.
So my questions are:
Do I need a 4bolt main
and
where is a good place to find one.
Not looking for nothing fancy. Just a good, new, strong 350.
any advice is welcome. Thanks in advance Laz