My 68 with a 69 351w. It's not that bad at all. The only pain is tightening header bolts, but with the gaskets that I use I haven't had a bolt back out in about 4 years. As for headers, I used the cheap Heddman 351w swap shorty headers and haven't had a single issue with them in the 7-8 years they've been there.
If you want hp and tq you should go with a 351 based motor. I never thought mine was hard to work on, even with a charge pipe in the way. I can't believe a 351 in a fox is any where near tight. There is less room in a 68, but I was never bothered by it, even when I was pulling all 8 sparkplugs repeatedly when I first started tuning it. Building headers was a bear though.
Maybe with an oval shaped log. Or maybe with a pin on hood and cut the apron out between the fire wall and shock tower and use shorties. Really though the shock towers would need to go IMO.
Ford used to offer a 392 crate motor and I believe it was based on a 351W block. If you think about it a crate motor comes w/ a guarantee and reliability. But ther are formulas to build a 302 and get 400hp for around 2500 USD. I'm talking old school 302 w/ cast iron heads. I think the guy's name was Joe Sherman.