Controllers really should not be dictating to you what kind of equipment your aircraft should or should not have.  The reality is that aircraft like the 707 and 727 are very popular in the Flight Sim community, and most controllers I've come across are going to be very understanding.  I used to fly the 727 regularly online for a virtual airline and had no problems.

However, it should be noted that it's perfectly possible to fly the same kind of routes that FMC-equipped modern aircraft fly with the equipment the 707 and 727 have.  All you need is the VOR and the DME and a chart, all an intersection is is the point where two radials from a VOR intersect.

So, basically, even if controllers give you guff about flying VOR to VOR, the only thing limiting you flying the same route as a 737NG is your own ability.  Get a chart, and start practicing flying to intersections.  It's not that hard, and you'll actually be flying like a real pilot, instead of just being a button pusher!