273 upgrade to 4bbl?

If you look at the Offenhauser ad above here you will see the same intake is offered for 64-5 273s and 66 and up 273 and 318 LA motors (with your casting number on the first of those.

so the simple answer to your question is no. The 1964-5 273 is the first of the LA motors, but the intake bolts were smaller and at a slightly different angle. otherwise they are the same. you could drill out the bolt holes but the angle of the intake that you are tightening the bolts down on would be slanted and you might damage the manifold or nor seal well. people have done it successfully but it is a risk. There are tapered washers of various angles that can be bought and I think the right one is a 6 degree taper, that can correct the mating surfaces.

The thing is that there are always people on here trying to find intakes for early 273s and they are hard to find. Intakes for any LA 318 are correct for your 273 and can be had cheaply and easily. Rather than butcher your vintage early 273 intake, why not put it up for sale here on FABO and find a 318 manifold that would actually work better - and given your application find an actual dual plane manifold. You might even pocket some green given how plentiful the later manifolds are and how rare your is.
@65dartcharger