I am running my 305 lg4 with HO Cast iron heads alongside a THM350 And 3.23 gears.
I am disappointed as I hit 85 MPH and I am redlining at 5200 rpm. I am looking to do some performance changes such as cam, different Carb (currently a Roch. Q-Jet Carb) intake and headers. So you can see I don't want to be limited to 85 MPH. So I am looking for changes in rear different gears and transmission. Would a 4 speed be better. Thinking of doing the manual swap. And if I were to do a 4 speed manual swap what gears would work better??

 

Gearing:
3rd in a TH350 is 1 to 1.
4th in a 4 speed is 1 to 1.

So you wouldn't gain speed from a 4 speed swap.

You'd benefit from a TH700-R4 (.70 4th gear) or 200-4R (.67 4th gear) or T5 (.63 or .73 fifth gear) and there are others. Getting a 700 or T5 from a junked third gen so you can get all related parts makes the swap easier than chasing all the parts.

Your early LG4 has lower compression than the L69 (or late LG4) and not much cam.

I've put a decent amount of miles on L69s and LG4s and done mods. I wouldn't cam yours. I'd build an engine that has a well-matched cam / heads etc. and swap it in with headers. The Q-jet is more than capable.

Do some more reading on here. Figure out your goals, plan your setup and go through it on here in detail.

 

Pretty sure you tach is not reading correctly. That speed/rpm is not correct for 3:23 gears

 

The HO L69 used the exact same identical #416 heads as the LG4. Same springs, same valve sizes, everything. So you can stop typing that. It just makes you seem clueless. With a 3.23:1 axle, 26" tires, and a non-lock torque converter, 85 MPH is around 3750 RPM. How are you getting 5200? Slipping trans? Bad tachometer? If it's stuck in second, the tach should show 5600 at 85, again assuming no lockup. Speedometer error? Switching to a 4-speed manual transmission would remove the torque converter slippage, so you'd gain another 5 MPH. Right now you have some diagnosing to do, rather than spending money on speed parts.

 

The only reason I made it apparent that I have the HO heads is that my lg4 doesn't match up with the date code for when they brought out the HO motoring mid year. All the casting numbers pointed to the power lacking 150 HP lg4 they brought out in the beginning of the '83 year.now I haven't delved in to my rear end that much so I am only going on the 3.23 gears as that is what I was told these cars came with from factory. So I could be wrong as I have now realized that there were, and don't quote me but 3.73 gears. Now I may sound like a Dumba** saying this but it is a 10 bolt rear end single tire fire. Probably doesn't tell anyone anything but thought it would be good to know. And I do know that I am pretty accurate because mg dads 89 grand was hauling *** at about 95 and climbing and I ran out of motor and couldn't keep up.

 

If you can run at the track, it may help in finding where you top out at,
along with top speed...

 

As soon as I get the windshield fixed aloń with rear main seal and new Trans pan and gasket I will be
Its just expensive to make a pass or two with my car just to find out how fast it does go.