[b-man]

I’m not sure but I’d say do a trial fit with a bare block with crank installed along with the heads, bolted to the long tail shaft Chevy TH400 you’ll be using.

Take note of the position of the crankshaft nose from side to side and from whatever reference point for the height before pulling the 389. Then drop the BBC package in and use the original trans mount on the crossmember to locate that end.

Chances are pretty good that a pair of BBC Impala engine mount frame stands could be bolted to the front crossmember after finding the proper location.

I can’t imagine that there’s such a huge difference that you’re going to have to splice in an Impala front frame stub. Both the Pontiac and BBC engines have a rear sump oil pan so the BBC pan shouldn’t interfere with the Pontiac engine crossmember.

If that all goes well I’d say a set of headers made to fit a 427 Impala should work or at least could be modified to work without having to completely remake them.

Just get a little adventurous and test fit it all is what I’d do.