This article shows my Tele-Chek substitute kit with a 5AXP4 CRT:
http://antiqueradio.org/5AXP4PictureTube.htm
As explained in the article, the 5AXP4 has a 53-degree deflection angle, making it suitable to sub for many early round CRTs. I don't know about your 8AP4 offhand, but if it has a similar deflection angle and it uses magnetic focus (and the same pinout), the 5AXP4 should sub.
Here's a website where you can look up the data sheet for your 8AP4:
http://tubedata.milbert.com/index.html
Regards,
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html