I may get flack for this, but if I was doing it I would hold out for a 350. I have ran 283's, 327's, 305's and 350's in various street rods, and really, the 305 is a weak sister. The physical outer dimensions are identical, but the heads don't "breathe" all that well and the horsepower gains that you will get from aftermarket equipment will add up to the cost of a 350.
Then again---I have driven stock flatheads, and even the sickly 305 is a vast improvement over them------and if your getting it free, well-------