I suppose, but one concern would be that the left spar contains the low mounted clutch and brake pedal pivot so it isn't easily moved. I tried to find a few pictures online illustrating how the spars look. I found some good ones, but the website doesn't seem to exist anymore, but you can se them still in Google's cache:
http://images.google.com/images?hl=en&lr=&ie=ISO-8859-1&q=+site%3Awww.classic-car-adventures.com+volvo+544+restoration

Good side view:

Not that this car has a 144 crossmember - and that brake booster is occupying the space normally taken up by the heater and blower equipment that sits out under the hood instead of in the dash as on a newer car.

Here's a B18/20 short block looking like it has plenty of elbow room:


I think that possibly the main conflict wouldn't be with the distance between the spars but the way they curve upwards near the firewall. The rectangular I4 engine doesn't have a problem, but the wide top end of a V engine probably would.