The C6s are great trannys but as far as the aftermarket has come with the C4, the C4 ends up being the better unit between the two. The C6 is a little bigger and much heavier than a C4. Personally, I'd go with an AOD, but if cash is an issue, just stuff in a C4 for the time being, like Dave was saying.
When looking for C4s, pay attention to where you're getting it from. You shouldn't have any trouble with the swap as long as you get the flexplate, heat shield, and Tq Converter from the tranny's donor car, it should be a snap. The C4 cam in a few different bellhousing configurations. There was the earlier C4 that used a 157 Tooth flexplate or the later 168 (i believe) flexplate both having the 10.5" converter bolt circle. Earlier C4s refer to the smaller more slanted 65-73 C4 trannys. The later C4s (74-79) and the 80-82 C5s had a slightly larger diameter flexplate and a bellhousing that bulged out more at the back against where the firewall would be. The earlier C4 bellhousing works the best in most streetrod apps and in most cases the bellhousing/TQ converters can be interchanged. There was also a HD C4 that has what's refered to as a "stepped case" where the bellhousing bolts to the case rather than to the front of the pump case like the typical C4s do. These "step case" trannys also typically have the dipstick going into the oil pan rather than into the case. These are commonly found in Pickups and Econoline Vans and such. I'd stay clear of these if you're space limited. This particular C4 has a much larger bellhousing and uses an even larger TQ Converter with an 11.4 bolt pattern (same as an AOD but different tooth count on the flexplate starter ring, I believe)

You mentioned messing with electrical on an AOD... What year was the AOD? I use the 89 and older (or was it 87??) Anyhow, these trannys only require a TV cable and that's it. No electrical garbage to fool with. Just a 3 wire plug for the backup/neutral safety switch. I definately know the carbed cars with an AOD were TV cable opereated. I used one out of an '87 Crown Vic in another rig I had and it worked great.
As far as the balance issue, 81-82 and older was 50oz imbalance and 80 and older was 28oz imbalance. The 351W through all the years was 28oz imbabance. Just use an AOD flexplate from a 351W and it'll work fine on the '77 302. Even a HD C4 flexplate may work as the converter bolt circle is the same, but I never compared the two flexplate to see if the ring gear tooth count was the same or not.

Be advised though, the AOD is considerably wider than a C4 (doubble the width pan compared to a C4) so be cautios of your exhaust routing an other things under the chassis.