71 Dodge Dart Swinger in England

Yes LA which stands for Light A engine which replaced the earlier heavier A engine. These went all the way up to the "magnum" V8 which started in about 1992. Later LA engines in the 1980s came with roller camshafts. All magnums have roller camshafts A magnum 318 or 360 is a direct bolt in replacement for your LA engine if you eventually decide to go that direction. Trans and motor mounts bolt up, exhaust manifolds bolt up, and your distributor will bolt up. All your pulleys, and FEAD "front engine accessory drives" will bolt up. Alternator attach point might need a little modification though bacause heads are different. To do a magnum swap you need a 4bbl iintake manifold with a magnum bolt pattern.

When you crack the throttle open very slowly and advance it very slowly does the engine smoothly pick up RPM without a stumble?

If so, look at your distributor. Check the vacuum advance to see if its working properly. Easy way is attach a length of clean vacuum line to the vac advance and pull suction on it with a mityvac or your mouth and watch to see the arm pull in and the timer plate move. Stick your tongue over the hole on the hose, or if using a mityvac watch the gage. if the gage shows it leaking down, or it will not even hold vacuum, then the diaphragm on the vacuum advance is ruptured.

Second thing to check is the mechanical advance. Under the timer plate theres a pair of mechanical advance weights and springs. If its a mopar electronic distributor under the distributor rotor theres a small felt pad. Every time the car is tuned up its supposed to get several drops of clean motor oil dripped on the pad. There is a c clip under the pad, and an internal shaft that works the mechanical advance. A lot of times people dont oil the shaft under the rotor and the shaft starts to bind or stick. An easy check is to turn the rotor while its on the shaft clockwise to check the mechanical advance it should move roughly between 1/8" - 3/16" when you let go of it, the rotor should snap back to its original position quickly. If it doesnt snap back, or moves back very slowly, then you probably need to take the distributor apart to unstick and lubricate the advance weights, and unstick the shaft under the rotor.

Not sure how much you know on ignition systems however this setup uses a combination of vacuum and centrifugal (mechanical) advance to keep engine time as the engine speeds up so you dont get a hesitation or stumble.