The username is ScottishDave, but I'm going to call you SuperDave because of the aggressiveness of your project!
My free advice, is to "eat the elephant" in small bites...
Yeah, if you look at all the problem spots at once it's easy to get depressed and spend the rest of the afternoon drinking beer in a bucket seat on the driveway...
Looks like you have a good opportunity to work from "front to back" on this Blazer since you are already headed that way. With the motor out and the front clip off you can do a nice job cleaning up the framerails all the way to the front cab supports.....wire wheel, degrease and shoot them with a nice Eastwood Chassis Black paint. Get the engine crossmember and front crossmember too. Clean out the nests in the heater system, and maybe put a fine wire mesh across the openings to keep the critters out next time...I've seen a few guys do that.
Clean and paint the firewall before the new engine goes in, then you won't have to worry about disassembling the whole truck again when you finally reach the paint stage. If you can get the firewall and cowl areas into the final color now, it will be simple to just tape them off later.
Looks like you need a new core support, if the inner fenderwells look salvageable, they can get the same Eastwood Chassis Black paint as the frame. By the time you're done up front the entire engine bay and frame section will be DONE, and you can focus on some rust repair in the footwells and rockers....just keep moving systematically toward the back of the truck, but try not to tackle the whole thing all at once. You need some smaller victories along the way to keep your motivation strong.
