Gopek,
I just converted my 70 from a SB to a BB, so I went out and measured the SB radiator. It was 28 1/4" W X 18 1/2" H not including the tanks. I had a radiator shop build a 4 core for me which is 29" W X 18" H. It bolted right up using the top cover. I did have to purchase the BB shroud however.
I agree with Pat, any reputable rad shop should be able to make one for you.
Hope this helps.
Mike H.
ACES # 1882