your 305 SHOULD have a roller cam- it's easy to verify without tearing too far into the engine. pop off a valve cover, and shine a flashlight down one of the pushrod holes and look down where the pushrod goes into the lifter. if the lifter has a retainer around it and the one next to it, with a big stamped steel "spider" across all 16 lifters, then it is a roller cam. if not, then it isn't.
as for switching distributors, you don't need to on stock Chevy cams. i know some will say you HAVE to, or your cam will be destroyed and your family will be cursed for 7 generations, but that just isn't true. at least not the first thing, anyways. i know from experience that you can run a good old junkyard hei with the stock gear on it on a factory roller cam- i did it for 2 summers on my 87 roller cam 350 before it got "upgraded" for duty in the Nova. i still have that same dist in it, only now i have the GM HOT hydraulic roller cam in it. i have had the distrib out more than a few times, and have seen no funny wear patterns of any sort, nor have i seen shiny metal flakes in the oil.
to sum up- the 305 is probaly a roller cam motor, and you don't have to switch gears.
you don't plan sincerity.
you have to make it up on the spot.
wanna hear about 20 years ago when i was too smart to know any better?