I did some quick measurements between my 82 K5, and 51 Wagon.
May be off an inch either way, but close enough to start making some plans..
Rear axle center to firewall: K5 87-88 in, Willys 81-82 in
Firewall to headlight buckets in Willys: 34 in
SBC rear to water pump bolts, long water pump: 29 in
Max width of radiator, at headlight buckets (to remain inside hood and trimming inner fenders): 34-35 in. Without trimming, 29.5 in
So I see why the build notes I found mention moving the motor back 6 in.
There should be plenty of room for a good size radiator in there, moved forward to almost touching the headlight bucket.