Cutting side entry hole in main breaker panel
You can put a hole in a NEMA-1 enclosure, but if it is a NEMA 3R (raintight, outdoors) the conduit entry must be able to maintain the N-3R rating. The most common fitting is called a Myers Hub. This assures you from dripping water on live parts and that's why KO's are at the bottom and lower sides. Also any sharp edge must be bushed. I'm assuming that's you "threaded union" (not an electrical term).

As far as bending radius of conductors here's a link that may help.
http://www.cooperindustries.com/content/dam/public/bline/Resources/Library/catalogs/meter_mounting_equipment/ring-type_metering/ed-minimumwirebendingspace.pdf

Its Tables 312.6 (A) &amp; (B) of the NEC. 

There's also NEC rules for conduit entry and bending radius that is Article 314.28 pretty much the whole thing. It may not be directly relevant but should be looked at for verification.