If your planning a swap anyways id go ahead do it and not waste your money on a new slant six transmission.
But imo I like the original slant six's in these early cars!
I've never done a V-8 swap in a 63, but the firewall in a 63 is one year only and may cause interference problems with the distributor on a small block.
64-66 firewalls are the same except for the size of the electrical junction, 64 is slightly smaller and 65-66 are a little larger. 64-66 also have a different
w/wiper set up.
