Yes, it can be done.
No, it's not a straightforward job.
If your 74 has had a change of front nosecone at some point to the earlier type, it will have the bigger holes for the Y-shaped bumper bar brackets. If so you can fit the Y-shaped bumper bar brackets in place of the strut-type fixtures easily.
OTOH 74 model nosecones have 2 square holes for the bumper struts to mount to the front bumper to the chassis rails. The old style Y-shaped brackets won't fit without chopping up the nosecone, and we don't want to do that. So it's necessary to cut the flanges on the front of the strut, where the commando bumpers mount to them, so they'll fit behind a pre-74 slim-line bumper. You'll probably need to fabricate a mounting plate too so that the bolts on the slim-line bumper can attach to the bumper strut flanges.
At the rear, the pre-73 slim-line bumpers used to attach to the very rear of the rear chassis rails on the lower beaver panel, via an angled bracket. But for starters, 1974 140s have no rear beaver panel.....
So to fit slim-line rear bumpers, again it will require cutting down the mounting flanges from the bumper struts, and fitting / fabricating a suitable mounting bracket to join the flange and the bumper.
Fitting slim-line bumpers is one of the (many!) jobs I still have to do for my 1974 142 project. I think I have all the parts here, if not, I have them on the mainland. If you want pics of the bits, please ask.