Plenty of room. The tube that is part of the swing arm is the same ID across the whole thing. The bronze bushings press in from either side and stop on the flange. If you took a third and cut the flange off you could press it to the middle. The pivot tube already rides on these bushings with the bolt inside. The third bushing would keep the bolt and tube assembly from flexing at mid point. The clearances for the outside bushings can be used on the third one as well.
Are you talking about the stock swingarm here? It is not the same ID all the way through...
My 2 cents' worth of info: here's the particulars on the OEM arrangement -
The clearance of the bushings is less than 0.0010" over the ends of the collar where they ride. I use 0.0004"-0.0008" (depending on straightness of the swingarm's tube) when making bronze ones. You can fit a flangeless bushing of up to 1.800" length inside the arm and still fit the seals and end caps.
The overall length of the outside of the [new] collar must be 0.001"-0.016" more than the length of the outside of the bushings and their seals, as tightening the bolt usually shortens it a bit. Too short, and it all binds tight when tightened.
The press-fit of the bronze bushings (as I make them) is 0.0004"-0.0006" into the arm, more will usually force you to hone the ID afterward from bushing crush during insertion.
The collar is narrower in the region between the bearings: it usually must be for assembly reasons. At the very least, the inside edges of the bushings must be beveled on both OD (to insert them) and ID (to help insert the collar, later). The collar OD in the bushing areas is either 0.8440", 0.8442", or 0.8444", depending on which collar you have.
The big bolt is 0.550" OD where it meets the narrower bearing zones of the collar: the collar is also 0.5515"-0.5520" ID at both ends, and in the center, to ride on the bolt in 3 positions, to reduce flex. The collars have a bearing surface in their centers to ride on the bolts at this location, too.
Installing a larger OD collar can be done by simply making one with the same ID size (and center support zone, like the OEM version), using the same bolt to stabilize it. Some have thus adapted Kawasaki and Suzuki swingarms to the 750 and 550 frames when looking for longer arms. These (that I have seen) have 1" ID sites where the (originally needle) bearings fit: thus the OD of the needle bearings was 1" size and a collar was made to fit the ID of these bearings, with a hole that fit the OEM bolt in the 3 support sites.
Most of the mods I have seen had more to do with longer swingarms than for stiffer arms: the early-style arm is rock-solid against flex (only the phenolic bushings in the OEM setup cause flex). The post-1975 style arm (with round tubes instead of box sections, welded together) are not as stiff.