I know I have been posting alot but Im new to this age of truck and need alot of advice!

I have been working on a 1947 GMC 248 for a few months now and its just not cranking the way I think it should.
I understand that 6v systems dont crank very fast but Ive got my new truck hooked up to it and every time I engage the starter it only turns over maybe a turn, sometimes not even that much. Even with the new truck turned on and running about 14 volts it just doesnt turn over very well. I dont know much history on the truck but what does it sound like to you guys?
Poor Connection?
Weak starter?
Something major in the engine itself?

Thanks in advance


Logan
2010 Silverado 2500HD
1947 GMC 300