Techinspectors overview of the reasons for the performance drop on these engines is correct, but I want to add a little bit for those of you to those of you who are tempted to compare factory HP ratings of the engines from the 60's to the HP ratings of the 70's engines. In 1972 (if memory serves correctly) the HP rating system changed from gross to net Horsepower. The result was an instant drop in HP numbers even though in some cases the engines were unchanged from the 71 versions. Comparing gross and net numbers is comparing apples and oranges.

The 70 429s did make good HP however they did it with either 10.5 to 1 or 11.3 to 1 compression (the specs tech provided). From personal experience trying to run this much compression on the street is not a lot of fun. If you do build the 429, I would strongly suggest keeping the compression down to 9 to 1 (stock compression on the 72 was 8 to 1).

Also for the 429/460 owners I want to highlight what Tech said ".....use a 68-70 timing chain and sprockets......." in a effort to reduce emissions Ford retarded cam timing on the 72 and latter 385 series engines with a resulting loss of performance. Dollar for dollar using the earlier timing set likely the most cost effective performance mods you can do for these engines.