You just ordered the wrong package type. The HC just means "high speed CMOS", but the package can be a standard 24 pin DIP or wide body style. The 74154 is also available in both standard and wide body DIPs. There is no adapter I am aware of, though one could be cobbled together. I would just order the correct part
http://www.marcospecialties.com/control/keywordsearch;jsessionid=BC9A673FD77E962E685E93F943C72CD8.jvm1?SEARCH_STRING=74154

Might not want HC parts either. the B ports on 6821s are TTL logic level which in some cases are controlling the 74154 decoders (Bally MPU).

Good part numbers

74HCT154
74LS154
74154

You want the N suffix (74HCT154N) to (dual in line through hole package) with 0.6" distance row to row (wide or unspecified)
The middle letters are family (hct = high speed cmos with TTL level inputs, typically a good choice for logic chips on old systems)

https://www.greatplainselectronics.com/proddetail.asp?prod=74HCT154

Is there a way to test that chip with a multi-meter, in or out of circuit?

Not easily.... I just checked my minipro programmer to see if it tested it and it's conspicuously absent. Maybe the other cheapie chip testers on the market could do it.

Multimeter would be too slow to catch inputs and outputs, logic probe could work. Generally if you put a logic probe on this type of chip and see some activity happening on most of the pins, the chip might be ok. To verify 100% you'd have to put it in a tester or make a test jig and simulate what's going on in the game to test it out. I suppose the multimeter could average out the signal to give some reading, in that case it would work - if you see a pin that should be moving always stay high or low, might be an issue.

I had read that it can be tested as any 74xx IC, so I did, and it turned out to be bad. I had somehow had a spare, I have no idea why, but I removed, socketed and replaced and my displays came back to normal.

ebay.com link: itm

I have one of these things. It is nice to diagnose a bad chip, desolder it out and have that thing declare it really was bad. That is the main way I use it. It tests most 40XX, 45XX and 74XX chips including 74154. I have found a few odd balls it cannot do but for the most part reads them all back.

Some limitations. It can tell between CMOS and TTL outputs but cannot tell difference between HCT and HC. It also cannot tell between or no letters, LS, F and L. It does not test 74S assume because that family is a power hog. Some specialty types like 7437 will read back as 74LS00 because the pinout/gate logic matches. Also does test LM339 I noticed.

As do I. I use it on all ICs for my complete boards before I insert them into a socket.

I have found this as well. One IC that I see a lot is 74LS245 and it can't seem to recognize this one.

Everything that barakandl said has been my experience. I find this tester invaluable.

there are few ancient and obsolete 4xxx ones on the SB300 it can't do. 4562 shift register and at least one more I can't think of. I normally trust a new IC from auth dist. is going to be fine, but nice to check ones where I am forced to use old stock or salvages out of China.

Did get a bad $5 lm2679t5 out of the tube from digi key tho, so i guess it does happen.