The engine compartment in your car was designed for a long, narrow, six-cylinder engine, laterally speaking. Not a lot of room from side-to-side.
One of the design feature requirements of the L-A 273 V8 when they were developing it, was that it be made narrow enough to fit the small engine bay in the Valiants and Darts with no problems, which it did.
The 318, 340, and 360 L-A (and later Magnum motors) have exactly the same external dimensions as the 273... unless you're an expert, you wouldn't be able to tell the differece by looking... they're ALL the same size, length, width and height.
So, as someone pointed out, if you bother to swap in a 273, you might as well make it a 360, because the swap difficulty (of which there is little) will be the same. Junkyard 360s are probably cheaper than 273's, anyway, because they are so plentiful; they came in everything. The 273's are getting rare... so, they might well cost more. There is no appreciable weight difference in any of the L-A/Magnum engines. Nothing you'd notice, anyway.
You would have to spend a considerable amount of money on a slant 6 to bring it up to the performance level of a bone stock 360 4bbl motor.
A 4bbl 360 with only a 340-style cam and headers in an early A body would easily run over 100mph in the quarter mile, which is probably faster than you want to go, anyway. You might not even need the cam to break 100...
You'd spend considerable cash trying to get anywhere near that kind of performance out of a slant 6... A similarly-cammed 318 in your light, early A-body car with headers, would easily be a high 90-mph car in the quarter-mile, so either would work as well, for your purposes.
If you care about sprightly (not "race") performance at all, the L-A or Magnum V8 swap would be my choice. Built-up slant 6's are cool, no doubt about that, but in the end, you'll have a faster, easier to accomplish, and cheaper car to "build" with a V8 swap, I think.
The early A-bodies don't require a "K-member" swap to make the switch to a V8, like the later ('67-up) cars do. Yank out the slant 6, and the V8 bolts right to the original slant 6 K-member just fine. The center link (steering) might have to be swapped for one with a depression for pan clearance, and you're going to need a 904 transmission with the V8 bell housing; the transmission that's in there only fits slant sixes. If your driveshaft has a ball-and-trunnion front u-joint, you might want to replace the driveshaft with one that has a conventional (more modern-design) front u-joint yoke.
The only other consideration is the headers, which is not my bag. Maybe someone else, who has knowledge about early A-body V8 exhaust systems can help us out, here.
I know that an exhaust header company called "TTI" recently came out with a great set of headers for this car with an L-A or Magnum V8 in it, but as usual, they're not giving them away. I have TTI headers on my own ('72) 360-powered Valiant and could not be happier with them, so maybe they're worth the money, but if you're on a budget, there's got to be a cheaper way.... Maybe Dakota/Magnum exhaust manifolds witll work for this application????? Anybody know?
If you do a cost analysis that includes even a smidgen of high-performance capability for this car, I think you'll find that the V8 is the more practical (cheaper in terms of both time AND money), and easier way to go.
The Poly will not fit this engine compartment without major surgery to the inner fender panels and shock towers; it's just too wide, and is a lot heaver. anyway. Not a good choice...
Good luck; this looks like a really fun project!!!
Hope this helps.
Bill, in Conway, Arkansas
