Get rid of the PG, 200R4 bolts in ,no driveshaft replacement/mod needed.
Relocate trans crossmember. Need to hook up trans TV cable correctly for trans to live & shift correctly.


200R4 will work better than a TH 350 ,lower 1st gear.
2.73''s,,,hmmmmm,,not good,,, unless you want to put in a SBC 400.
2.73's w/a 250 6 will not be quick N/A (normally aspirated)
Gear change will be a must. 3.42-3.73 & a 200R4 trans

300 HP,easy w/a turbo,not easy N/A worn out engine.
You will need to rev it to make 300 HP,not going to make 300 HP @ 4500 RPM.

I do not like Hollys either.
Edelbrock AFB or AVS good choice.

Run a small chamber 194 cyl head to bump up compression 1 point or use your stock 250 head (stock compression 8.5:1) & these pistons TRW L 2289 .030 over forged pistons for chevy 250.No chamfer
https://www.inliners.org/ubbthreads/ubbthreads.php?ubb=showflat&Number=61714#Post61714
Try & find SBC 307 cast pistons,4 valve reliefs w.no chamfer on top sides.
Stock rods, side grinded/polished & shot peened w/ ARP rod bolts.
Cast iron manifolds :
http://stoveboltengineco.com/index.php/c...product=1395980
Cyl head work will cost the most ,lump ports,larger valves etc.
use http://www.sissellautomotive.com/
or Larry AKA Twisted6: http://www.t6racing.org/

Run a stock style 8" HP SBC damper & have a machine shop machine the "V" groove/s in the damper to drive the accessory/s
Example: http://cgi.ebay.com/ebaymotors/SBC-Chevy...sQ5fAccessories
Or one of there 6.3/4" to keep the drive ratio close to stock.

Camshaft, .480-.500 lift 220 degrees duration @ .050 in. & ex. lobe center of 114.
Stall on converter 2200-2600 with Lock-up depending on cam & compression you end up with. you get

Realy quick & dirty run down off the top of my head.


MBHD

12 port SDS EFI