Does not look like many miles on the motor, with the cross marks still showing in the bores and how clean the motor is. You have a great starting point. I had and sold a 13 spline A-body, 833 transmission. Looks like you have a 23 spline E or B body one. Yes, you need to get a 13 spline one. As like the 340's all the Hemi blocks casting numbers are the same. The heads you have are from a later Hemi 66 and up. COOL find!!!!!