Try to get the casting numbers on the block to be sure what ya got. Thats the real way of knowing, or take the old starter in and have it matched up. I had what I thought was a 350 and had a 400. the starter didnt fit either. I didnt realize it until I ran the casting numbers that I had the 400. The housings are different. Just my 2 cents. Find the casting numbers, they do not lie.
Oh and 2 more cents, go to NAPA.