300ci Straight 6 power upgrades


New Member
Jul 27, 2021
I have a 68 Fastback with an inline 6 300 and a manual. the car I believe had a 351 Windsor originally but a previous owner replaced it with the 300. ive decided to build the I6 because ive had a truck with one before and really love straight 6s anyway. This car is bone stock other than that and I was wondering if I could get some advice on how to get more power and maybe gas mileage out of it and what parts/companies to use. Im on a budget a little but ive got a few grand to put into it. I got a Exhaust manifold off of a 71 dumptruck that i read elsewhere were the best for these particular engines if you plan to turbo them down the road. The head needs redone so wanted to send it off the get rebuilt and possibly get it ported, port matched and polished but wanted to see if there are any alternative heads/ upgrades i should do first. Thanks in advance for any help.
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wanna catch the space herp
10 Year Member
Sep 1, 2010
I wish I could refer you to a magazine article, but that’s about a done deal.
If this is the right episode, they got a 300 I6 from an industrial machine and took it from 88 HP to at least as fun as a base 289/302 should have been. I do not remember if they added a turbo to it in a later episode.


5 Year Member
Apr 16, 2015
9 means 69, which explains the M for engine code.

I would say it is a swapped VIN if the car looks like a 68. What does the door tag say?

Also, where is your VIN plate mounted? 68 was passenger side dash, 69 was driver side dash.
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