0.3v drop across the OEM GP bus is probably OK. IMO, it's not a very heavy bus, and with
two spade connectors in series between the GP Relay and the GP bus, it's amazing that it works as well as it does.
Spade terminals are just not up to the kind of load put on them in that circuit, over the long term.
Buy the largest CCA you can afford and that can fit in there -- remember, several diesels (even Fours) have two batteries paralleled to get enough CCA to start them.
And for Gawd's sake, stay away from Exide batteries. I like and buy only Interstate, and the Mega-Tron Plus is their top-of-the-line (until you get to the AGM batteries (like "Optima"), which are a different line altogether).
Using Interstate's online app. guide, I see both the MTP-27 (750 CCA) and MTP-78DT (800 CCA). I have a couple of the MTP-27s in my two Maxima Wagons, they're good but they're also long. The MTP-78DT is 1.5" shorter, which will fit much better in the 720.
This afternoon, I stole the MTP-27 out of the '83 Wagon to start the '82 720 ("Bellingham Truck") to move it off the tow dolly and into the shop, prepatory to dismantling it. The MTP-27 will fit, but it doesn't leave a lot of room above the IP.
There is no wiring harness in the truck, I sold it to Andy in AZ last month. But the battery cables (ground and to starter) are still there, as is the basic GP bus. The truck and the battery have both been sitting outside and are at about freezing. The truck hasn't been started since about April.
I bolted on the terminals, put a light clip lead on the GP bus and connected it to the battery. The clip lead is really too light for this duty, but I let it stay connected while I fiddled around connecting a longer clip lead to the starter. Got that hooked up, thought I'd better make sure the starter still works, so I tapped it to the battery. Yup, the starter works, and the damned thing started. I hadn't had the GPs running maybe 20 seconds. I was surprised.
Anyway, Interstate batteries aren't cheap, but they are very good. Search this site for "Interstate" for more info.
MTP-78DT - Mega-Tron Plus - 85 Months - 800 CCA
"DT" = Dual terminal, ie both top and side terminals.