You have a couple options. There is no "bigger" k-frame. Its all mount related
First: You can use the Schumacher mounts, some of which require cutting a drilling but its minor. should be bolt in deal with a early abody.
2nd: cut off your mounts and use an engine plate you can but there is no real need to..
3rd: Build/Find a Direct Connection Big Block K frame wont work in an early a
4th: Upgrade to tubular K (RMS, Gerst, HDK etc) nothing wrong with that.. :)
Schumacher mounts will work just fine.