Another one where I'll take the Truck, lol. As much as I can tell, it is either a '62 or '63 GMC 5000 with a V-6 Gas Engine, Chrome Trim package to boot. Also has non-original type Cab Lights that are typically found on an International of that time period.
Likely a 351 or 401 CID. 302 CID engines were in the light duty pickups and medium box trucks, more or less. I call the GMC industrials "coffee can" engines because of the diameter of the pistons.
Right on the 351 and 401, but the small one was 305. Starting in '62 the 6500 had a 478. Then there was the famous 702 (more or less 2 351's "welded" together, lol). The V-8 version of this type of engine and what also replaced the 702 in '66 was the 637. Besides the smaller trucks, I used to drive a '62 with the 478 and a '71 with the 637. Both of these had very huge pistons as you have stated.
What an odd collection of 75 model year GM products, on top it looks to be a Ventura, Malibu Wagon, Rally Sport Camaro, Sunbird coupe, and a Nova LN? Lower deck a Impala Sedan, Regal Coupe, Astre (for some reason I don't think its a Vega), and a Century Sedan.