350 motor doesnt spin freely by hand with transmission on

Hey hotrodders,
I am in the process of cleaning up a 350 motor. Not rebuilding or anything just making it look nice. It is completely stock. When i brought the motor i made sure i could spin it manually by hand and made sure it wasnt locked. Seemed to feel like it had good compression. The motor belonged to a buddy of mine so i knew what i was getting, just checked it cause it was sitting outside and didnt want a hydrolocked motor. Anyways I got the motor home with the th350 converter my friend gave me an slapped on the th400 transmission I had laying around. As soon as i mated them together the motor was very hard to spin. EXTREMELY HARD. So i took off the tranny thinking the converter didnt sit right. This time i made sure it sat 2 times. Bolted block and tranny and still was really hard to turn. I made sure the tranny was in park and i also tried it in neutral but with no luck. I took off the tranny one last time sat converter on again and tried it and still no luck. My QUESTION IS WHY WONT THE MOTOR SPIN FREELY WHEN I SIT TRANSMISSION ON!! It isnt the starter, i took it off. Also tried without spark plugs and still a no go. If i crank the motor by using the starter it cranks 3 times then kills the battery. It could be a bad battery. Its just been sitting. Well if yall have any questions I'd be glad to answer them. Thank you, any suggestions are good suggestions