Your well into your conversion and too bad you don't live closer to me, I have an extra GMC 1 ton frame that would have been lots less work.
GMC 63-66 came only with leaf springs and my 14 bolt 3.42 geared rear out of a 79 diesel GMC bolted right in.
The front suspension did too.
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Building Genuine Pontiac Firepowered cars and trucks for several decades.
Why Pontiac?? That's just me daring to be different.
Projects..
63 C10
Short
Wheel
Base fleetside (The Pro-Street) currently under reconstruction.
440CID 2X4 Pontiac