Cranking amps needed?

[motorhead0320]

I am new to this forum and to the boat owner world as well. The other day I purchased a 1974 ranger bass boat with a 70hp evinrude. I hope to restore it but i first need to get the thing running well. I know hte motor runs and I gotten it to start a few times. I am very unfamiliar with the starting procedure for outboards.
The first thing i really need to know is what cranking battery size i need? I purchased a cheap one from advanced auto but it only has 575 and I dont think that has enough for my size motor. But, what to I know, thats why I am asking you guys.
Good news is the boat does float. I got it to start up for me, but after getting out the middle of the lake it stalled on me. By the time i played with the choke, throttle, and tried to get it started the battery had no more life to make the starter spin. That made for an embarrasing low in from a couple guys on the lake. Good thing I had that stupid WHISTLE.
I think i need to take the thing in for a tune up. I am big JEEP guy which stands for Just Empty Every Pocket, I guess whatever change i have left will be in this boat because it seems like it will be the same time type of hobby.
Any suggestions about batterys or the corrects starting procedure would be great. Sorry this post was so long. Thanks in adavnce.