If your looking for rugged dependability,,then the 300 six is the best ford motor out there,,(just my humble opinion) my brother has a 96 and his is a 300 efi,,there's lots of room until you get to the firewall, the last plug is a bit hard to get too, but not impossible.
If your looking for extreem measures,,then get yourself a good 460 cube V8,,the gas mileage sucks,,but the shear grunt those 460's give ya is something else.
The suggestions about lifting the cab are good ones if you got the room to do it,,but make sure you tag every plug from the firewall to the motor,,and the tranny to firewall,,before you go disconnecting and lifting the body.