BA fly from Heathrow and US Air from Gatwick (and I think, but not sure, Manchester) non-stop to Philadelphia. So if your friend is in Philadelphia itself that may be a better option. If you do go to New York remember there are 2 airports served from London; JFK and Newark. I reckon Newark would be more convenient for Philadelphia than JFK. Newark is servde by BA and Virgin from Heathrow and Continental from Bristol, Gatwick, Birmingham, Manchester, Glasgow, Edinburgh (I think that's the complete list). If they live south of Philadelphia BA fly to Baltimore as well.
The 767 is a comfortable aircraft. I just flew one two weeks ago Heathrow/Baltimore. I'm going home in 2 days on a BA 777 Washington/Heathrow which is even bigger and more stable. Size does tend to matter but even a 747 will get juddered by bad turbulence. All I can say is the flight out was smooth.
You may want to investigate "Premium Economy". BA and Virgin offer it as a separate class; bigger seats, more legroom. I'm 6'2" but I'm not tubby (my BMI is just about OK!) and BA "World Traveller Plus" works well for me. I find ordinary eceonomy (what BA call World Traveller) cramped.
The other thing to bear in mind is that there are daylight flights from Washington, JFK and Newark back to Heathrow. I find them much more comfortable than spending 8 hours trying to sleep in any seat (even First Class!). It's also perfectly possible to fly Heathrow/Philadelphia and return Newark/Heathrow - you don't have to fly to and from the same airport.
I know nothing about Maxjet or Silverjet or EOS. Sorry.