CHEAPest way to convert rear axle bolt pattern to big bolt pattern???

Cheapest and easiest way into a large bolt pattern axle for your "A" body?

No brainer here; just do a little research so you know what to look for (starting here was good) like just about any '73 to '76 7 1/4" or 8 1/4" rear axle from a disc brake "A" body for instance, then network with all your Mopar community aquaintances and buddies, and don't forget to diligently peruse Craig's List! You'll turn something up, and in the case of the usually unwanted 7 1/4", for cheap, free, or nearly so.

Anything else you do (drilling axles, modifying brakes, adapting a "B" body Mopar or one of the Ford axles) is just about guaranteed to cost you more time, money, effort, and/or a combination of one or more.