Welcome.......
Build the V6. It would be good for autocrossing. Just a tad less weight and since you probably wouldn't get out of second gear, the power output of the V6 would be spot on.
Now here's the part everyone expects from me......... Sorry guys.......
Do an all aluminum V8. No, not that expensive one.
'61-'63 Buick/Olds 215 cubic inch V8 from the Buick Special/Skylarks, Pontiac Temptests and Olds F-85s. Requires some motor perches(I think) with V6 mounts, and a special bellhousing that can be had from D&D Fabrications(
www.aluminumv8.com ). They have all the info and conversion pieces on the shelf at their shop for swapping one into an H-body.
200 HP out of a small displacement V8 and it weighs 1/2 of a SBC @ 320 lbs ..... complete.
Great for handling. You really don't have to upgrade the springs or brakes. It weighs about 30 lbs more than a stock Vega 2300......... No appreciable weight gain, double the HP with 240 ft-lbs of torque. Fun fun fun.......
Aftermarket parts are readily available since Rover bought the rights to the engine in '66 and have basically been running aluminum Buick engines since..........
See my '72 with a 215 in it in My Garage...... click on link under my picture.... or see it in My Garage........ or check it out right here on my cardomain site....
www.cardomain.com/ride/2202359
Here's a few articles on the history of the 215 as well as performance upgrades........ look at the one titled
"Vega V8 Swap".... it shows pics, too.
http://lotus.www.50megs.com/V8articles.htm
OK...... I'm done. I've got more info if you want.