If the vacuum has changed AND, the timing has changed, AND it takes a lot of throttle to achieve rpm, then in all liklyhood, the Timing chain has jumped.
There is an easy way to check chain stretch; just crank the engine over by hand until she meets resistance, Then crank it the other way. First it will move easy, but then, suddenly, as the slack is taken up, it will get hard again. If you cannot feel it, then pop the Distributor cap off and watch the rotor. If the crank moves more than 4 degrees with no reverse motion of the rotor, go to plan-B
Plan-B
With Solid lifters there is an easy way to check this.
1) First prove that the timing marks on the damper are correct to within a degree or two.
2) remove whichever valve cover is easiest. If the Driver's side, go to the front cylinder, if the passenger side, go to #6.
Crank the engine over until the chosen cylinder is making compression, bring it to TDC, then manually crank it over exactly one revolution.
3) this puts her on the overlap cycle where one valve is just opening and the other is just closing. Shuttle the crank back and forth a few degrees looking for a position where both valves are approximately the same amount open, When you find it STOP. read your timing. It should be in the window of TDC>6 or 8 degrees BTDC.
4) if it is then the cam is in pretty close.
5) but if it is not, then the cam is out of phase
AFAIK the original factory 273 top sprocket was all steel and those are hard to jump a chain on.
But the nylon-capped 318 sprocket only lasts reliably to maybe 90,000 miles, and then it tears all those nylon caps off, or sometimes just 25% of them cuz the engine quits running.
