IMs have used CB Performance Type 1 longblocks for years, then dressed them themselves up in Vancouver. I've never had any issues getting the plugs out, but my setup was supplied by me, not by CB or IM. If you are having issues, it's probably a result of interference between the intake manifolds and/or the cylinder tin. There are no shortage of intake manifolds that need a little bit of grinding to allow a plug wrench on the plugs, and the tin is a one size fits all proposition that (in truth) generally fits a stock engine pretty well and needs a bit of tweaking to work with anything bigger. If you need to open up the tin, you'll probably need the engine on the floor. The intake can be removed and ground, if that's where the interference is.
I can almost guarantee that your engine came to IM with the plugs in, and that they installed the intake, tin, and plug wires afterwards. Their plug socket might have been thinner walled than anything you've got. You mentioned needing to drop the engine to replace the plugs before. If that was the case, and you are still having problems-- then your mechanic was also part of the problem. It's not a big deal, once everything is out in the open to make it all work nicely. No remove the tin or intake, replace the plugs, and just put it back together without fixing the interference issue is lazy. I'd personally have a bigger issue with an ongoing problem perpetuated by a mechanic without the initiative to fix the problem than an initial (and understandable) problem from the builder.
If this were me, I'd grind a socket down as thin as it can get, and try again. I've done this before. Most plug sockets are enormously thick. If you can't move the mountain, move Mohammed. If this won't do it, you'll need to follow the instructions in the above paragraph.
Your plug gap is going to be a factor of a lot of different things. You've got the CB MagnaSpark and a Bosch coil. The gap with that setup is going to be different than if you had a CDI box or some hotter spark. Your wires will also factor into this-- are they 7mm resistance wires or 8mm copper core? They could also be 8mm resistance wires and plugs, so look at the jacket (and the plugs) to see. I would assume they are 8mm resistance wires and resistance plugs. If that's the case, .028 would be a safe maximum. .024 would work too, if everything else is right. Look at the plugs when they come out-- chocolate brown is perfect, and I wouldn't change a thing. Black but not sooty is generally OK too, as most people set up the jetting a bit rich. Black and sooty (or worse, wet) means you need to be jetted leaner or get a hotter spark, or both.
Good luck. Forewarned is forearmed.