1963 Signet 200

Unsure about differences in heater boxes, I've never compared the two. Both are under the dash so I doubt it is a big obstacle. The firewall is slightly different on a 63 compared to a 64-66, as mentioned above. The wiper motor moved slightly (still under dash) and the later firewalls had a small depression for distributor clearance which could be added with hammer if needed. The V8 cars had cable throttles, and slant cars had rod linkage, so a 63 firewall would need to be drilled for a cable. The other engine compartment difference is the radiator support. 63's only had 19" radiators, while the later cars could accept common 22" radiators. The hole can be made bigger to fit a later radiator with a saw.

Otherwise standard early-A swap issues like drop steering center link, and exhaust hassles apply.