I've done two superchargers, a Magnusen kit in my vette (put on the original stock engine, before the 427) and the Procharger on the 427 now. Both cost about $7500-$8,000 by the time they were in the car and re-tuned and everything was sorted out. I assume that one for a 305 would be a bit less costly, but not by much: I'd guess $6000-$6250 by the time its running perfectly. The SC is quick to install and effective: it adds about 33% capacity so in effect the 305 would perform like a 400 cube engine. Big difference.

The SC is what I'd do if I didn't want to mess with it for long and wasn't sure if I'd keep it long. On the other hand. . .

Displacement provides simplicity, low eng torque beyond even what a SC can provide, and a platform for a SC later, if you want REAL power. There are three routes.
1. stroker short block, decent but not great set of heads, and everything else for a 383 or 400 -- about $6000 in the car and sorted out.
2. shop around for a discount crate motor -- I would think you could get a really good 383 or 400 LT1 type for $5500 complete. Leaves only a little left out of a $6K budget, but its a crate motor, so there not much left to buy.
3. budget, find a nice LT1 or LT4 in a surplus yard with a 90-day guarrantee - ought to cost about $2500 which leaves plenty for headers, intake, etc. - even a cam before its put in the car.