advice on changing 440 timing chain

Fire your mechanic and take the majority of the posts as bad advice.
Your engine needs the timing chain replaced and who knows what else. $1500.00 is a lot of labor unless under the hood is a train wreck.
On your low compression 1975 440, the engine is "free-wheeling" because the pistons are too low in the cylinders to hit the valves. Pistons will not hit the valves if the timing chain breaks or drops because of nylon teeth breaking off. With your engine, total destruction is nearly impossible from a timing chain problem.

Vacuum Advance:
I disagree with All this stuff about limiting total timing to 45° with vacuum advance.

For my 9.2:1 compression 440 with factory style electronic ignition the I bought from Rick Ehrenberg.

Without vacuum advance connected: 34° total timing advance at 3000 rpm

I reconnected the vacuum advance hose to the distributor vacuum canister. Using my dial back timing light, I found 55 – 56° before top dead center at 3000 rpm. Rick Ehrenberg’s specification is 50° to 56° before top dead center. The reading is within his spec and 3000 RPM simulates a high-speed steady cruise. Cruise is where vacuum advance helps most, especially with fuel economy. Ehrenberg is an expert. I have driven 100s of miles with the same vacuum advance setting. I have had no detonation or timing issue of any kind.

Further, your mechanic who does not want to connect the vacuum advance does not know Mopar street cars. I agree with all who advise hiring someone else.
OK enough fear, I'm going to change the chain and I hope I can pull it out of the garage and onto ramps without incident.

This has given me more reason to make my next car electric. No timing chains, oil pans, gas tanks or other stuff to worry about. .I'm thinking an electric motor can go 1 million miles with out incidence. Paul

The odds favor you being able to drive it where you need to. 20 yrs ago I advocated electric vehicles with great fervor. I still like them, IFF done according to MY specs, though, alas, NOT as the corrupt swine running the automotive industry on this planet have it. I prefer to electrify an old vehicle, like one our C bodies for example. There is AMPLE ROOM for batteries and a motor, plus the control circuitry.

I hope you don't think an EV comes much safer than a combustion powered vehicle though. FAR from it! ANY ENERGY DENSE DEVICE STILL ABOUNDS IN DANGER! Electric motors can go catastrophically awry just as badly as internal combustion engines can. They require cooling, just as internal combustion engines do. Go look at the pure electric cars from Nissan as one example set. Propelling 1.5 to 2 tons of metal at say, 90 kph with inverted AC motors requires considerable cooling, hence, a liquid cooled radiator to keep the coils running right, and the bearings cool. Should the coils overheat, they either will quench, shutting the motor down, or ignite whatever insulation contains their conductors, if a good overcurrent protection system fails to cut the current in good time.

Spinning motor shafts can still cause much damage.

I assert that EVs are probably safER than combustion powered vehicles, but "safe," NO! All that charge storage if suddenly ruptured can still make a nasty BOOM! also.

Of course. the computer SLOPware these modern corporations insist on infesting their work with just CRAWLS with "bugs."

Yet even animal drawn vehicles were dangerous enough. Read John Hay's "Tales of Pike County" for stories of how pre-automotive vehicles crashed. burned, and mangled american bodies. And the chariot races of those Romans! Don't EVEN get me started on those......
aside from the "turn the engine till the rotor turns" method a lot of times if you put a timing light on it at idle and the mark seems to be dancing all over the place its an indication of a bad chain