Yep,If your swapping from a 6 to a V8 you need conversion mounts unless you have a v8 parts car. I would also upgrade the 9" brakes and axle too. That 7 1/4 won't last long
The problem comes when you try to go racing with them...if it's a stock 318, or even a mild one, the 7 1/4" will live until he finds a suitable replacement. I can't wait to replace the one in my '72 either, but it has 90K miles and I've not encountered a moment's trouble with it.
OP: There's an A body 8 1/4" down in H-town in the classifieds for $300 I'll see if I can find the link for you.