Shock tower clearance will likely be a problem in the 68, I'd thinkWhen we had the Cleveland in the Granada, it was a bear to work on and I'm betting a 68 isn't alot wider between the towers.
I believe they put FEs in those from Ford and I don't know if the front end on those cars was any different, but even if not, the Cleveland is wide at the outside of the heads. Headers are hard to install and plug removal will require taking the valve covers off.
In KVs 69, both of the above tasks are a hassle but easier than the old Granada.
Personally, I'd put the heads on the block and set it in the car to see what it looks like.
You didn't mention what trans. you are using, but I'd not use the 6 banger one. If its a C4 maybe overhauled, but I wouldn't trust any of the stock driveline components very far.
It sounds like you have a good idea of what else needs to be upgraded.
Good luck, it should be a cool setup.
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63 Falcon 377 Cleveland stroker Flying Toilet alchohol injection. 6.19 @ 110 MPH 1/8 mile
2002 Ranger FX4 daily driver
2000 F350 PSD Crew cab dually - Like commuting in a B52!!
98 US Cargo Phantom II 28'