First of all, join the Grand Banks owners association Grand Banks Owner's Resources
and ask your questions there. There are very few GB owners on this forum, and most of them participate in the GB owners forum, too. There are people on that forum with decades of experience with these boats, including the former owner of a yard that specializes in the repair, maintenance, upgrading, and restoration of GBs and people who have owned GBs for decades, some of whom restored their boats from something almost ready for the scrapyard.
We have one of the first batch of fiberglass GB36s ever made. Our boat was made in late 1973. One slip away from us is the last GB36 ever made, and it was made in the very early 2000s.
The GB42 went out of production in the mid or later 2000s.
The original molds for the GB36 and the GB42 wore out in the mid-1980s so they made new molds for both boats. The boats from these molds appeared in 1988, at least for the GB36.
American Marine took advantage of this to make both boats a wee bit longer, a wee bit wider, and significantly taller. What this means is that the post-1988 boats have more user space inside and more headroom. The forward head in our boat, for example, is the size of a phone booth. In the post-88 GB36 (we used to charter a '91 boat so have experienced the later boat) the forward head is larger and even accommodates a curtain-shower.
Grand Banks are NOT open ocean boats. Yes, they can be taken across open water if the conditions are nice. But they are a bit top-heavy with regards to dealing with big seas and waves, their windows are WAY too big for holding up in anything real rough with lots of water coming aboard, and their semi-planing hulls are not the most stable in the world for rough ocean conditions.
Grand Banks are considered, and were designed to be, coastal cruisers. That is one reason why they are more popular in the Pacific Northwest than any other part of North America. With 1,000-plus miles of protected and fairly protected water between Puget Sound and Glacier Bay in SE Alaska, they are, in many people's eyes, the perfect boat for this region.
But an open-water boat they area not unless one is able to restrict travel to days when the sea conditions are mild.
For open water work, a displacement boat is far more suitable. So Krogens, Nordhavns (expensive) and other such designs come to mind.
With regards to range, it all depends on what kind of engines one has, how many, and how fast one wants to cruise. We have two engines that at the rpm we use drive the boat about 8 knots and burn about 5 gph total. We carry 400 gallons of fuel. So that's 80 hours of run time. So that's a theoretical range of 640 nautical miles.
But that's never what happens in real life. Wind, current, startup, warmup, slow running in harbors and whatnot all conspire to reduce that range considerably, particularly up here where tidal currents area typically 3 to 5 knots and farther north in the islands can get up to 10 to 15 knots locally.
In a newer GB42 with the typical Cat engine installation of a pair of 400+ hp engines, fuel consumption at 9 knots will be some 6 of 7 gph, and at 15 or 16 knots will be as much as 25 gph.
But the bottom line as far as I'm concerned, is if your primary objective is to run hundreds of miles in open ocean in a slow boat that can't outrun the weather and the risk of getting caught out in rough seas is very real, a Grand Banks is not the boat you want.
Below are a couple of photos I took of our old '73 boat up in the islands.