Exactly what bellhousing is important if you are using the original mounts and clutch linkage. It's far less important if you are not keeping the stock rear crossmember.
If your crossmember is what I expect it will have flat surfaces where the bellhousing bolts to it. There is no V-8 bellhousing that fits that crossmember. In '54 the crossmember changed to angled mounts. That same style mount was used for years afterward, and the V-8 bellhousings for the later crossmember (starting in '55) will fit the '54-'55 1st crossmembers. They are not hard to find, I bought one at a swap meet for $5.00 a year ago.
All that to say that if you can score a '54 or '55 1st crossmember, you can then easily bolt the '55 to '67 (I think?) bellhousing to the crossmember and readily support a V-8. Clutch linkages vary and I am not the one to spell out those details.
All that said, with the 2.73 gears I'd consider running the Powerglide or maybe a Turbo 350 instead of the original 4-speed. I have an SM420 (what yours should be, provided its a 4-speed) in my '55 1st GMC, with a 270 and 3.08 gears.
Shifting from granny 1st to 2nd isn't really very driveable, and starting in 2nd sometimes I wish it had deeper gears. As small blocks go, a 307 has relatively more torque, but I'm wondering if you will be happy with it and those 2.73 gears.
Back to the bellhousing and crossmember, if you go with a later car type manual transmission or the Powerglide or Turbo 350, you can use the transmission mount instead of the bellhousing as the rear engine mount, and some of your problems go away.
Running an automatic with a torque converter the concerns about the 2.73 gears go away also.
