if you can get an 80s 350 block (or any year for that matter i think??), you will literally be able to drop it in with very little modification. the 350 is the same physical size as the 305, just bigger bore and stroke, so motor mounts, accessory brackets, manifolds, hoses, etc. should all bolt up no problems.

now i myself have never done this, but knowing that the blocks are the same tells me everything should be pretty straight forward. someone correct me if i'm wrong.

 

all small block chevys externally are the same dimensions and mount in the same location. so all the brackets mounts and accesories will work. except for lt and lsx stuff. i wouldnt want to go with a big block in a gbody car. you can make just as much power with a good small block with out the weight. big blocks just look good in the engine bay and are bassy as hell both good things. i wouldnt run drag radials on it with the 7.5 rear ive dynamited them with street tires. id also want to know what heads are on the car, was the motor rebuilt, does it already have an aluminum intake, 4 bbl carb,aftermarket cam, does it have a posi,from there you can formulate a better plan. if they didnt rebuild it and it has a million miles on it a power adder is out the cylinder pressure is just going to kill the motor. and i wouldnt do a cam with out doing at least bearings in the car. i say this cause ive done it before and spun bearings trying to cut corners. on my 80 when i got it. it had the 400 in it with a 2 barrel iron intake,exhaust manifolds,single exhaust, thm200 trans ,open rear with 2.73s. with an intake carb headers and duals with no cats, a 350 trans with 2200 stall it held its own. and with 3.73 with a posi it held its own real well stop light to stop light. just make sure before you buy it that you know exactly what your getting. heres a site with decent prices i buy from.
http://www.northernautoparts.com/