Welcome to the GTO forum !
You aren't the 1st to post in the late model section, & won't be the last.
(1) You can use the early block as long as you use 2-bolt engine & frame mounts. I've read that, at least some, '70-'72 455 GTO's came with 3-bolt engine mounts, which required matching frame mounts.(I suppose some call the parts that bolt to the block, engine or motor mounts, & the parts that bolt to the frame, either engine stands or brackets. I suppose everybody can call 'em whatever they choose.)
(2) IF the 6X heads actually CAME on a 455 engine, from the factory, the chambers are much too big for a 400 cube build. You could make 'em work on a stroker, especially a 4.5" stroker. Now 6x-4 heads, which came on either 350 or W72 400 engines, work great on low compression 400 builds. These and 6x-8 heads are very popular on 400 block stroker builds, with a 4.21 or longer stroke. Look for a small stamped number on the vertical pad, which is to the left of center, looking at the exhaust port side of the head. Maybe the number on yours will be either a 4 or an 8.
(3) Wouldn't even consider an Edelbrock carb, UNLESS it is a Q-jet, made by Weber, & sold for several years as an Edelbrock. I have a 1904 E-Q-jet on my 455 bracket engine.
(4) I'd swap the balancer, timing cover & all pulleys & brackets over to the early '70's type. The later model stuff is easier to find. I'd buy a new balancer. A Powerbond PB1056N is said to be a good quality replacement. That's what I have.
(5) Will most likely need to be bored at least .030". If you stay with the stock stroke, I'd go with Speed Pro L2262F forged pistons, in the bore size you need. You can either have the stock cast rods resized, using ARP brand bolts, or buy new forged rods. I recommend forged. If you decide on a stroker assembly, they come with aftermarket forged pistons, either flat top or dished, as well as forged rods.
(6) Cam choice will depend on lots of factors, including compression, cubic inch, & several other things. The most basic & still popular small 400 cam is an 068 Clone. Melling SPC-7 is probably the most popular of these. The Summit 2801 is also very popular. It is similar to an 068, but has more lift. If you go with more stroke, a Summit 2802 might be more like what you want. I chose this cam for my 455 bracket engine, because I'm on a VERY low budget.
(7) Most '67-'74 350, 400, 428, & 455 engines used the same or very similar flexplates. BUT, some were dual-pattern, and some were single pattern. Some factory torque converters required the smaller bolt circle, and some required the large bolt circle. Some aftermarket converters are dual pattern, so that they'll work with any flexplate.
(8) Well, there's obviously LOTS more that can be said. But, I'd let a GOOD machine shop check out the block/crank & let you know if it's rebuildable. AND, check the heads out. You need to know chamber size, to know if you can use 'em or not. If you can, then the shop will need to check 'em for cracks & see if they are usable. Make SURE that the shop you choose has experience rebuilding Pontiac engines. MANY Chevy shops do NOT.
If your block cannot be rebuilt, here are some options.
400 Long Block
455 Long Block
400 Short Block
455 Short Block
Good Luck !