Original 307 or drop in a 350?

[odis.dunkle]

So I am about to haul the father-in-law's '69 GMC over the Cascades to start its resuscitation.
It currently has a 307 V8 in it with a known problem--per my f-i-l, when its valve push rods were replaced apparently the seals weren't replaced and let oil leak by and the spark plugs would get messed up after a couple of weeks of driving. Rather than take it back to the mechanic, he parked the truck for 20 years.

So in preparing to start work on the truck, a deal fell in my lap for a 350 engine with a quadrajet carb. The engine is out of a '74 Nova owned by a former mechanic who just this past week upgraded his engine and carb to something with 'more kick'.
[numbers off the block = 13L11780, T0915CMA]

So now I am looking for opinions/facts on how I should proceed.
Keep in mind that I am looking for a 'frequent driver' that can still be used for hauling when needed. This is a long bed 3/4ton.
Also, I am paying someone else to do the work. (I apologize to the board for being contrarian in this way.)

The basic quandary is do I just move straight into dropping in the 350, or do I make an effort to repair the 307?
The truck as it sits is very much 'original', so do I try to preserve that?
Or do I go with the masses which seem to gravitate toward the 350?

Gas mileage? Resale value (both of the truck and either engine as a separate sale)? Reliability? ??
As I understand it from my reading/research, the Nova engine is the same block as the existing 307, so will the mounting and connections be equivalent?

The photos below show the engine compartment and the carb.
I would appreciate it also if someone can identify the existing carb under the dust.